HTML Compliance Checker

Every place your markup isn't strictly 100% to the HTML Living Standard, with the exact example and the correction.

One URL → five views. Enter below once, switch tabs to flit between audits.
Modern browsers render almost any HTML, no matter how loose, so broken markup often looks fine and ships. This tool flags every deviation from the HTML Living Standard: obsolete elements, unquoted attributes, self-closed non-void tags, missing alt text, duplicate IDs. Shares the audit cache with the other SEO tools.

Strict errors

Missing DOCTYPE, duplicate IDs, self-closed non-void elements, missing alt, missing <title> or lang. A conforming validator rejects these.

Obsolete features

Everything in HTML Living Standard §15: <center>, <font>, align, bgcolor, cellpadding, frameborder. Rendered but flagged.

Style / non-minimal

Uppercase tags, XHTML-style <br />, redundant type="text/javascript", unquoted attributes. Valid HTML5 but not what the spec writes.

About this tool

Modern HTML parsers are extraordinarily forgiving. Unquoted attribute values, <div/> instead of <div></div>, obsolete <center> wrappers, align="center", missing alt. Browsers silently compensate, the page renders, and the mistake ships. W3C's Nu validator catches these; this tool surfaces the same class of issues against the HTML Living Standard on any public URL, with a concrete example from the page and the exact correction.

The rules split into three buckets. Errors are strict violations: a conforming validator rejects them outright. Obsolete flags anything listed in HTML Living Standard §15 (<font>, bgcolor, frameborder). Style is the lint-pile: valid HTML5, but inconsistent with the spec's minimal form (uppercase tags, <br />, disabled="disabled").

For the authoritative answer, run your page through the W3C Nu HTML Checker. This tool is the fast iteration loop that shows each issue alongside the fix, not just the error.