Security & SEO

HTML Validation Checks

Validate your HTML against W3C standards and fix structural issues.

Updated 2026-03-15

2 min read

HTML

validation

W3C

HTML Validation Checks#

LemWatch validates your pages against W3C HTML standards and flags structural issues that can affect SEO, accessibility, and browser rendering.

HTML errors — Unclosed tags, missing required attributes, invalid nesting

Warnings — Deprecated elements, redundant attributes, accessibility hints

Fatal errors — Malformed markup that prevents proper parsing

Info messages — Best practice suggestions

Impact Area	How Invalid HTML Hurts
SEO	Search engines may misinterpret your content structure
Accessibility	Screen readers rely on valid HTML for navigation
Browser rendering	Different browsers handle errors differently
Performance	Invalid markup can trigger additional parser work

For each page checked, you'll see:

Error count — Issues that violate the HTML specification

Warning count — Non-critical issues and best practice suggestions

Fatal errors — Critical issues highlighted separately

Line numbers — Exact location of each issue in your source code

Missing alt attributes — All <img> elements need alt text

Unclosed tags — <div> without </div> causes rendering issues

Duplicate IDs — Each id attribute must be unique on a page

Missing lang attribute — <html lang="en"> helps accessibility and SEO

Deprecated elements — <center>, <font>, <marquee> should be replaced with CSS

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