# SEO — Glossary of Checks
30 kontrol optimalizácie pre vyhľadávače
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## S1 — Title Tag
**What is it:** Checks for the presence of an HTML tag on the page. The title tag is one of the most important on-page SEO factors, displayed in search results as the main link heading.
**Why it matters:** Without a title tag, search engines cannot properly display your page in results. Google uses the title tag as the primary signal for understanding page content and displaying it in SERPs.
**Real-world example:** Amazon.com uses precise title tags for every product, e.g. '<title>iPhone 15 Pro Max - Apple Smartphone | Amazon.com'. Pages without a title tag can lose up to 30% of organic traffic.
### Sources
- [HTML Element](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/title) — MDN Web Docs
- [SEO Starter Guide](https://developers.google.com/search/docs/fundamentals/seo-starter-guide) — Google Search Central
---
## S2 — Title - Length
**What is it:** Checks the length of the title tag, which should be 50-60 characters. A title that is too short does not utilize its full potential in search results, while one that is too long gets truncated with an ellipsis.
**Why it matters:** Google displays approximately 50-60 characters from the title tag in search results. An optimal title within this range maximizes visibility and click-through rate (CTR).
**Real-world example:** Wikipedia literally optimizes the title of every page. For example, 'Bratislava - Wikipedia' (22 characters) is short but effective. Conversely, a title with 90 characters gets truncated in Google and the user cannot see the full information.
### Sources
- [HTML <title> Element](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/title) — MDN Web Docs
- [SEO Starter Guide](https://developers.google.com/search/docs/fundamentals/seo-starter-guide) — Google Search Central
---
## S3 — H1 Heading
**What is it:** Checks whether the page contains exactly one H1 heading. The H1 is the main heading of the page, which tells search engines what the page is about. Multiple H1s or no H1 at all are problematic.
**Why it matters:** The H1 heading is the second most important on-page SEO element after the title tag. It helps search engines and users quickly understand the main topic of the page. Exactly one H1 per page is the established standard.
**Real-world example:** Google.com always uses exactly one H1 for every article on its blog. Pages with multiple H1 headings confuse search engines about the content hierarchy and can lower rankings.
### Sources
- [SEO Starter Guide](https://developers.google.com/search/docs/fundamentals/seo-starter-guide) — Google Search Central
- [Lighthouse SEO Audits](https://web.dev/articles/pass-lighthouse-seo-audit) — web.dev
---
## S4 — Heading Hierarchy
**What is it:** Checks for proper heading hierarchy from H1 through H6 without skipping levels. Correct hierarchy means H1 -> H2 -> H3 without jumping levels (e.g., going from H1 directly to H3).
**Why it matters:** Proper heading hierarchy helps search engines understand the structure and relationships between content sections. It also improves accessibility for screen readers and the overall usability of the page.
**Real-world example:** MDN Web Docs uses flawless heading hierarchy on every documentation page. Conversely, pages that jump from H1 to H4 lose semantic context and search engines cannot properly interpret the content structure.
### Sources
- [SEO Starter Guide](https://developers.google.com/search/docs/fundamentals/seo-starter-guide) — Google Search Central
- [Lighthouse SEO Audits](https://web.dev/articles/pass-lighthouse-seo-audit) — web.dev
---
## S5 — Canonical URL
**What is it:** Checks for the presence of a canonical tag (<link rel='canonical'>), which specifies the preferred version of a page. It helps prevent duplicate content issues.
**Why it matters:** Without a canonical tag, search engines may index multiple versions of the same page (with/without www, with/without trailing slash, with UTM parameters), which splits link equity and can lower rankings.
**Real-world example:** Shopify automatically adds canonical tags to all product pages so that URLs with filters (?color=red) do not take ranking away from the main product page. Without this, Google could index hundreds of duplicates.
### Sources
- [Consolidate Duplicate URLs](https://developers.google.com/search/docs/crawling-indexing/consolidate-duplicate-urls) — Google Search Central
- [HTML <link> Element](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/link) — MDN Web Docs
---
## S6 — Viewport Meta Tag
**What is it:** Checks for the presence of a meta viewport tag that ensures proper page rendering on mobile devices. The tag <meta name='viewport' content='width=device-width, initial-scale=1'> is the standard.
**Why it matters:** Google uses mobile-first indexing, which means the mobile version of the page is primary for ranking. Without a viewport tag, the page displays on mobile as a desktop version scaled down to a small screen.
**Real-world example:** All modern web frameworks (Next.js, Nuxt, Angular) add the viewport tag automatically. Older websites without this tag are flagged by Google as 'not mobile-friendly' and lose positions in search results.
### Sources
- [HTML <meta> Element](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/meta) — MDN Web Docs
- [Lighthouse SEO Audits](https://web.dev/articles/pass-lighthouse-seo-audit) — web.dev
---
## S7 — Robots Meta Tag
**What is it:** Checks whether the page contains a meta robots tag with a 'noindex' value, which prevents search engines from indexing the page. Verifies that main pages are accessible for indexing.
**Why it matters:** A meta robots tag with a 'noindex' value completely excludes the page from search results. It often happens that developers forget to remove noindex after transitioning from a staging environment to production.
**Real-world example:** A well-known case: a large e-shop forgot to remove 'noindex' after migrating to a new server and lost 80% of organic traffic within a week. Google Search Console can alert you to this issue, but with a delay.
### Sources
- [Robots Meta Tag Specifications](https://developers.google.com/search/docs/crawling-indexing/robots-meta-tag) — Google Search Central
- [Google Search Essentials](https://developers.google.com/search/docs/essentials) — Google Search Central
---
## S8 — Open Graph Tags
**What is it:** Checks for the presence of Open Graph meta tags (og:title, og:description, og:image, og:url). These tags determine how content appears when shared on social networks like Facebook, LinkedIn, and others.
**Why it matters:** Without OG tags, social networks generate previews automatically, which often results in an unattractive display. Properly set OG tags can increase click-through rates when sharing content by up to 50%.
**Real-world example:** The New York Times has precisely configured OG tags for every article, including og:image with an optimized preview image. When shared on Facebook, the article appears with a large image, title, and description.
### Sources
- [The Open Graph Protocol](https://ogp.me/) — Open Graph Protocol
- [SEO Starter Guide](https://developers.google.com/search/docs/fundamentals/seo-starter-guide) — Google Search Central
---
## S9 — Twitter Card Tags
**What is it:** Checks for the presence of Twitter (X) Card meta tags (twitter:card, twitter:title, twitter:description, twitter:image). These tags control how content appears when shared on the X platform (formerly Twitter).
**Why it matters:** Twitter Card tags enable richer previews when sharing links on X. They support various formats such as Summary Card, Summary with Large Image, and Player Card for video content.
**Real-world example:** Spotify uses the Twitter Player Card for sharing songs, so users can preview music directly on X. Without Twitter Card tags, only a plain link without a preview image would be displayed.
### Sources
- [Getting Started with Cards](https://developer.x.com/en/docs/x-for-websites/cards/guides/getting-started) — X Developer Platform
- [Cards Markup](https://developer.x.com/en/docs/x-for-websites/cards/overview/markup) — X Developer Platform
---
## S10 — Structured Data
**What is it:** Checks for the presence of structured data in JSON-LD or microdata format (Schema.org). Structured data helps search engines understand page content more precisely and display rich snippets.
**Why it matters:** Pages with structured data can earn rich snippets in results (star ratings, prices, FAQ, recipes, events). These enhancements increase visibility and CTR by up to 30%.
**Real-world example:** Amazon uses Product schema on every product, allowing Google to display price, availability, and ratings directly in results. Recipe sites like AllRecipes use Recipe schema to show preparation time and ratings.
### Sources
- [Introduction to Structured Data](https://developers.google.com/search/docs/appearance/structured-data/intro-structured-data) — Google Search Central
- [Google Search Essentials](https://developers.google.com/search/docs/essentials) — Google Search Central
---
## S11 — Image ALT Texts
**What is it:** Checks whether all images on the page have a filled alt attribute. ALT text describes the content of an image for search engines and assistive technologies (screen readers).
**Why it matters:** Search engines cannot 'see' images and rely on ALT text for indexing. ALT texts are also crucial for web accessibility and are displayed when an image fails to load.
**Real-world example:** Wikipedia consistently uses descriptive ALT texts for all images, e.g. alt='Bratislava Castle at sunset'. E-shops without ALT texts miss out on traffic from Google Images, which can account for 10-20% of visits.
### Sources
- [SEO Starter Guide](https://developers.google.com/search/docs/fundamentals/seo-starter-guide) — Google Search Central
- [Lighthouse SEO Audits](https://web.dev/articles/pass-lighthouse-seo-audit) — web.dev
---
## S12 — Internal Links
**What is it:** Checks whether the page contains at least 3 internal links to other pages on the same website. Internal linking helps search engines discover content and distribute link equity across the site.
**Why it matters:** Internal links are the foundation of search engine crawling. Pages without internal links are 'orphaned' - search engines struggle to discover them and do not assign them sufficient authority.
**Real-world example:** Wikipedia is a master of internal linking - every article contains dozens of links to related pages. Thanks to this, Google can efficiently crawl millions of pages. A blog without internal links can lose up to 40% of its potential ranking.
### Sources
- [SEO Starter Guide](https://developers.google.com/search/docs/fundamentals/seo-starter-guide) — Google Search Central
- [Google Search Essentials](https://developers.google.com/search/docs/essentials) — Google Search Central
---
## S13 — External Links
**What is it:** Checks for the presence of external links to trustworthy sources. Linking to high-quality external sources increases the credibility of the page and helps search engines understand the topical context.
**Why it matters:** External links to authoritative sources signal to search engines that your content is grounded in a broader context and is trustworthy. Google perceives this as a content quality signal.
**Real-world example:** Scientific articles always reference sources and studies. Similarly, a quality blog post about health should link to WHO or professional sources. Pages without external links can appear isolated and untrustworthy.
### Sources
- [SEO Starter Guide](https://developers.google.com/search/docs/fundamentals/seo-starter-guide) — Google Search Central
- [Google Search Essentials](https://developers.google.com/search/docs/essentials) — Google Search Central
---
## S14 — Anchor Text Quality
**What is it:** Checks the quality of anchor texts (link texts). Anchor texts should be descriptive and relevant, not generic like 'click here', 'here', 'more', or 'read more'.
**Why it matters:** Anchor text helps search engines understand the content of the target page. Descriptive anchor texts like 'SEO optimization guide' are far more useful than 'click here', both for users and search engines.
**Real-world example:** Google uses descriptive anchor texts in its own documentation, such as 'see our structured data guide'. Conversely, email marketing campaigns often use 'click here', which is a missed opportunity from an SEO perspective.
### Sources
- [SEO Starter Guide](https://developers.google.com/search/docs/fundamentals/seo-starter-guide) — Google Search Central
- [Lighthouse SEO Audits](https://web.dev/articles/pass-lighthouse-seo-audit) — web.dev
---
## S15 — Clean URLs
**What is it:** Checks whether the page URL is clean and readable - without unnecessary query parameters, session IDs, or random strings. Clean URLs are easier to remember and share.
**Why it matters:** Clean URLs improve user experience and help search engines understand the site structure. A URL like '/products/nike-air-shoes' is better than '/p?id=38291&cat=12&sess=abc123'.
**Real-world example:** Airbnb uses clean URLs like '/rooms/12345' instead of complex query parameters. WordPress with 'pretty permalinks' enabled changes '/p?id=123' to '/article-title'. Clean URLs have demonstrably higher click-through rates in search results.
### Sources
- [SEO Starter Guide](https://developers.google.com/search/docs/fundamentals/seo-starter-guide) — Google Search Central
- [Google Search Essentials](https://developers.google.com/search/docs/essentials) — Google Search Central
---
## S16 — HTML lang Attribute
**What is it:** Checks for the presence of the lang attribute on the HTML element (e.g., <html lang='en'>). This attribute defines the language of the page content for search engines and assistive technologies.
**Why it matters:** The lang attribute helps search engines serve the page to the correct language audience. Screen readers use it to select proper pronunciation. Without it, search engines may serve the page to the wrong audience.
**Real-world example:** A Slovak website without lang='sk' may be shown by Google to German or Czech users. Global companies like IKEA set the lang attribute on every language version of their site (lang='sk', lang='cs', lang='de').
### Sources
- [HTML lang Global Attribute](https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/lang) — MDN Web Docs
- [Lighthouse SEO Audits](https://web.dev/articles/pass-lighthouse-seo-audit) — web.dev
---
## S17 — Favicon
**What is it:** Checks for the presence of a favicon - a small icon displayed in browser tabs, bookmarks, and search results. A favicon increases brand recognition.
**Why it matters:** Google displays the favicon next to the URL in mobile search results. A favicon increases the credibility and recognizability of a website. Without one, a generic icon is shown, which lowers CTR.
**Real-world example:** GitHub uses its iconic octocat favicon, which is instantly recognizable among ten open browser tabs. Websites without a favicon appear less professional, and users have a harder time finding them among open tabs.
### Sources
- [HTML <link> Element](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/link) — MDN Web Docs
- [SEO Starter Guide](https://developers.google.com/search/docs/fundamentals/seo-starter-guide) — Google Search Central
---
## S18 — Status Code and Broken Links
**What is it:** Checks the HTTP status code of the analyzed URL and verifies all links on the page. Every <a href> link is actually verified with an HTTP HEAD request to determine whether it is functional or broken (404, 5xx, timeout).
**Why it matters:** Broken links damage user experience and SEO. Google lowers the ranking of pages with many non-functional links. The page's HTTP status code must be 2xx - any other status (e.g., 5xx) indicates technical issues.
**Real-world example:** An e-shop has 85 links on its homepage. After checking, 3 lead to discontinued products (404) and 1 external link points to a partner company that no longer exists (timeout). Fixing these links improves crawl budget and user experience.
### Sources
- [SEO Starter Guide](https://developers.google.com/search/docs/fundamentals/seo-starter-guide) — Google Search Central
- [HTTP Status Codes](https://developer.mozilla.org/en-US/docs/Web/HTTP/Status) — MDN Web Docs
---
## S19 — Robots.txt
**What is it:** Checks for the existence of a robots.txt file in the root of the website. This file tells search engine crawlers which parts of the site they can and cannot crawl.
**Why it matters:** Robots.txt is the first file a search engine crawler reads when visiting a website. A properly configured robots.txt optimizes crawl budget and protects sensitive sections of the site from indexing.
**Real-world example:** Facebook has an extensive robots.txt that blocks crawling of profiles but allows public pages. An incorrect robots.txt can block the entire site - a well-known case where 'Disallow: /' accidentally blocked an entire e-shop for weeks.
### Sources
- [Robots.txt Introduction and Guide](https://developers.google.com/search/docs/crawling-indexing/robots/intro) — Google Search Central
- [How Google Interprets robots.txt](https://developers.google.com/search/docs/crawling-indexing/robots/robots_txt) — Google Search Central
---
## S20 — Sitemap.xml
**What is it:** Checks for the existence of an XML sitemap, which provides search engines with a list of all important URLs on the website. A sitemap speeds up the discovery of new and updated content.
**Why it matters:** A sitemap is like a map of the website for search engines. It is especially important for large sites, new sites with few external links, and sites with rich multimedia content.
**Real-world example:** CNN.com has a sitemap index with links to sitemaps for news, video, and images. WordPress generates a sitemap automatically. New websites without a sitemap may wait weeks for Google to discover all their pages.
### Sources
- [Sitemaps Overview](https://developers.google.com/search/docs/crawling-indexing/sitemaps/overview) — Google Search Central
- [Robots.txt Introduction](https://developers.google.com/search/docs/crawling-indexing/robots/intro) — Google Search Central
---
## S21 — HTTPS
**What is it:** Checks whether the website uses the HTTPS protocol with a valid SSL/TLS certificate. HTTPS encrypts communication between the browser and server and is a ranking factor for Google.
**Why it matters:** Google has used HTTPS as a ranking signal since 2014. Browsers mark HTTP pages as 'Not Secure', which deters users. HTTPS is now an absolute baseline for every website.
**Real-world example:** Let's Encrypt provides free SSL certificates and most hosting providers offer HTTPS automatically. Websites without HTTPS lose user trust - research shows that 85% of users leave a site marked as 'Not Secure'.
### Sources
- [Google Search Essentials](https://developers.google.com/search/docs/essentials) — Google Search Central
- [Lighthouse SEO Audits](https://web.dev/articles/pass-lighthouse-seo-audit) — web.dev
---
## S22 — WWW Consistency
**What is it:** Checks whether the website consistently uses either the www or non-www version and properly redirects one version to the other. Both versions should not work simultaneously without a redirect.
**Why it matters:** For search engines, example.com and www.example.com are two different pages. Without a redirect, link equity is split between both versions, which weakens the overall site ranking.
**Real-world example:** Google.com redirects www.google.com to google.com (without www). Conversely, www.amazon.com is Amazon's primary version. The important thing is to choose one variant and redirect the other via a 301 redirect.
### Sources
- [Consolidate Duplicate URLs](https://developers.google.com/search/docs/crawling-indexing/consolidate-duplicate-urls) — Google Search Central
- [SEO Starter Guide](https://developers.google.com/search/docs/fundamentals/seo-starter-guide) — Google Search Central
---
## S23 — Trailing Slash Consistency
**What is it:** Checks for consistent use of trailing slashes in URLs. The addresses /page and /page/ are different URLs for search engines and should be unified.
**Why it matters:** Inconsistent use of trailing slashes creates duplicate URLs, which confuses search engines and splits link equity. The site should use one convention and redirect the other.
**Real-world example:** Next.js allows setting trailingSlash: true/false in its configuration. Apache servers often add trailing slashes automatically for directories. It is important to have a consistent rule and a 301 redirect for the alternate variant.
### Sources
- [Consolidate Duplicate URLs](https://developers.google.com/search/docs/crawling-indexing/consolidate-duplicate-urls) — Google Search Central
- [Google Search Essentials](https://developers.google.com/search/docs/essentials) — Google Search Central
---
## S24 — HTML Size
**What is it:** Checks the size of the HTML document, which should not exceed 100 KB. Overly large HTML slows down parsing, rendering, and increases page load time.
**Why it matters:** Large HTML documents slow down page loading, consume more memory, and can cause issues on mobile devices with limited data plans. Google favors fast pages.
**Real-world example:** Single Page Applications (SPAs) sometimes generate HTML with inline CSS/JS exceeding 500 KB. Google recommends keeping HTML under 100 KB. WordPress themes with page builders often generate unnecessarily large HTML with dozens of superfluous div elements.
### Sources
- [Lighthouse SEO Audits](https://web.dev/articles/pass-lighthouse-seo-audit) — web.dev
- [SEO Starter Guide](https://developers.google.com/search/docs/fundamentals/seo-starter-guide) — Google Search Central
---
## S25 — Mobile-Friendly Indicators
**What is it:** Checks basic mobile-friendliness indicators: viewport tag, responsive design, readable font size, sufficiently large touch targets, and no horizontal scrolling.
**Why it matters:** More than 60% of searches happen on mobile devices. Google uses mobile-first indexing, meaning the mobile version of the page is primary for evaluation and ranking.
**Real-world example:** Google offers the Mobile-Friendly Test for testing mobile-friendliness. Pages that are not mobile-friendly lose positions in mobile search results. Responsive design is today's standard - Bootstrap and Tailwind CSS handle it automatically.
### Sources
- [SEO Starter Guide](https://developers.google.com/search/docs/fundamentals/seo-starter-guide) — Google Search Central
- [Lighthouse SEO Audits](https://web.dev/articles/pass-lighthouse-seo-audit) — web.dev
---
## S26 — Content Length
**What is it:** Checks whether the main page content contains at least 300 words. Short content cannot sufficiently cover a topic and search engines may consider it 'thin content'.
**Why it matters:** Pages with little content (thin content) have lower rankings because they do not provide sufficient value to users. Google prefers comprehensive, valuable content that thoroughly covers a topic.
**Real-world example:** Studies show that pages in Google's first position average 1,400+ words. Wikipedia articles with thousands of words dominate search results. Product pages with only 2-3 sentences lose organic traffic compared to competitors with detailed descriptions.
### Sources
- [SEO Starter Guide](https://developers.google.com/search/docs/fundamentals/seo-starter-guide) — Google Search Central
- [Google Search Essentials](https://developers.google.com/search/docs/essentials) — Google Search Central
---
## S27 — Keyword Density
**What is it:** Checks the density of keywords in page content. Too high a density (keyword stuffing) is penalized, while too low means the page is not sufficiently relevant for the given keyword.
**Why it matters:** Google penalizes keyword stuffing - artificially repeating keywords. A natural density of 1-3% is optimal. Modern search engines use semantic understanding, so synonyms and related terms are equally important.
**Real-world example:** The old SEO approach of 'cheap shoes, shoes cheap, buy cheap shoes' is penalized today. The Google Panda update in 2011 started penalizing keyword stuffing. Quality content uses keywords naturally in context.
### Sources
- [Google Search Essentials](https://developers.google.com/search/docs/essentials) — Google Search Central
- [SEO Starter Guide](https://developers.google.com/search/docs/fundamentals/seo-starter-guide) — Google Search Central
---
## S28 — Hreflang Tags
**What is it:** Checks for the presence of hreflang tags on multilingual websites. The hreflang attribute informs search engines about language and regional variants of a page (e.g., sk, cs, en).
**Why it matters:** Without hreflang tags, Google may display the Czech version of a page to a Slovak user or vice versa. Hreflang ensures that the correct language version is shown to the right audience.
**Real-world example:** IKEA.com uses hreflang for dozens of countries: hreflang='sk' for Slovakia, hreflang='cs' for the Czech Republic, hreflang='de-AT' for Austria. Without hreflang tags, Google could show the German version to Czech users, leading to a high bounce rate.
### Sources
- [Localized Versions of Pages](https://developers.google.com/search/docs/specialty/international/localized-versions) — Google Search Central
- [HTML <link> Element](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/link) — MDN Web Docs
---
## S29 — Pagination (rel prev/next)
**What is it:** Checks for proper use of rel='prev' and rel='next' on paginated pages. These attributes inform search engines about the relationship between pages in a sequence (e.g., page 1, 2, 3).
**Why it matters:** Proper pagination helps search engines understand that multiple pages form a logical whole. Without it, paginated pages may be perceived as duplicate content with similar titles and descriptions.
**Real-world example:** E-shops like Alza.sk use pagination for categories with dozens of products. Properly set rel prev/next helps Google understand that /shoes?page=2 is a continuation of /shoes. Google marked rel prev/next as a 'hint' rather than a directive in 2019, but still accepts it.
### Sources
- [Consolidate Duplicate URLs](https://developers.google.com/search/docs/crawling-indexing/consolidate-duplicate-urls) — Google Search Central
- [HTML <link> Element](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/link) — MDN Web Docs
---
## S30 — Breadcrumb Navigation
**What is it:** Checks for the presence of a breadcrumb navigation element on the page. Breadcrumbs show the hierarchical position of a page within the site structure (e.g., Home > Category > Product).
**Why it matters:** Breadcrumbs improve navigation, reduce bounce rate, and Google displays them directly in search results instead of the URL. Structured data for breadcrumbs (BreadcrumbList schema) increases visibility in SERPs.
**Real-world example:** Amazon displays breadcrumbs on every product page: 'Electronics > Computers > Laptops > Gaming Laptops'. Google shows these breadcrumbs in search results, helping users understand the page context before clicking.
### Sources
- [Breadcrumb Structured Data](https://developers.google.com/search/docs/appearance/structured-data/breadcrumb) — Google Search Central
- [SEO Starter Guide](https://developers.google.com/search/docs/fundamentals/seo-starter-guide) — Google Search Central
---