Markup Validation
Markup validation is checking a web page's code against the rules of the language it's written in, usually HTML, to catch mistakes like tags that never close. The W3C, one of the groups that sets web standards, runs a free Markup Validation Service for this. The W3C also says passing validation isn't a full quality check.
Source: W3C Markup Validation Service, About, checked October 29, 2026
What Markup Validation means for your business
A perfect score isn't the goal for search. Google's page on page metadata says, "Google tries to understand HTML even when it is invalid or inconsistent with the HTML standard, but errors in the markup can cause problems with how your metadata is used in Google Search."
The errors that matter most sit in the head of the page, the hidden section at the top that holds the title, description and other tags. Google says that once it finds an element that doesn't belong there, it "assumes the end of the <head> element" and stops reading the rest. So an image or embed placed inside the head can stop Google from reading a canonical or robots tag that comes after it.
Structured data gets its own check. Google's Rich Results Test validates that code and previews some search features, which is a separate job from the W3C validator.
We'd run the validator, fix the errors in the head first, then anything that breaks headings or links, and leave the long tail of small warnings alone. If Google doesn't seem to use the title, description or canonical you set, checking the code at the top of your pages is a fast place to look.
Source: Google Search Central, Use valid HTML to specify page metadata, checked October 29, 2026
Last reviewed October 29, 2026. All terms
How AI answers it today
On October 29, 2026, Google's AI Overview cited a code quality guide, Reddit, Wikipedia, a web services firm and YouTube, and said valid code helps crawling without citing Google. ChatGPT gave an HTML example, linked the W3C validator and didn't mention search.
Our run through ChatGPT and Google's AI Overview, October 29, 2026. A sample, not what every buyer sees.
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