SEO — 30 checks | Check Glossary

S1

Title Tag

What is it

Checks for the presence of an HTML <title> 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</title>'. Pages without a title tag can lose up to 30% of organic traffic.

Verified sources

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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

Verified sources

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.

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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.

Verified sources

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.

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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').

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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.

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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.

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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.

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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.

Verified sources

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'.

Verified sources

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.

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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.

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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.

Verified sources

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.

Verified sources

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.

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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.

Verified sources

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.

Verified sources

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.

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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.