Free W3C validator alternative: markup and CSS conformance in one pass
Conformance matters not because a badge says so, but because browsers recover from broken markup by guessing — and they do not all guess the same way.
By Sapun Lamichhane · Arcetis
Markup validation has an image problem. It is associated with badges from 2004 and with pedantry, and plenty of successful sites carry hundreds of errors without visible consequence.
The case for it is narrower and better than the badge suggests: valid markup is the only version of your document that every browser interprets identically.
What browsers do with broken markup
They recover, using error-handling rules that are themselves specified — and then they diverge, because recovery involves choices. An unclosed element gets closed somewhere. Where depends on the engine.
The practical result is a layout that is correct in one browser and subtly wrong in another, with nothing in your CSS to explain it. Anyone who has spent an afternoon on a bug that only appears in one browser has plausibly spent it on this.
What conformance underpins
- Accessibility. Assistive technology reads the parsed structure. If the parse is wrong, the announcement is wrong.
- Structured data. Markup nested inside the wrong container is markup attached to the wrong thing.
- Answer engine extraction. An assistant bounding a passage uses your element structure to decide where it ends.
- Your own scripts. querySelector against a document the browser restructured returns things you did not expect.
Raw response or rendered DOM?
Both, for different reasons, and this is where the classic validator falls short on a modern site.
Conformance is properly judged against the raw response, because that is the document the specification describes. But the structural problems that affect real users are frequently in markup JavaScript created after load — and a validator fetching your URL never sees any of it.
What free means here
Ours checks HTML and CSS conformance in a single pass, free, with no account for your first scan. It reports against both the raw response and the rendered DOM, and orders findings by consequence rather than by document position — so the unclosed element generating forty downstream errors appears above the deprecated attribute that affects nothing.