Parsing
Markup must be valid: complete start/end tags, no duplicate IDs, no duplicate attributes, properly nested elements. (Note: WCAG 2.2 deprecated this criterion as obsolete; modern browsers no longer require strict validation.)
What it requires
WCAG 2.0 SC 4.1.1 was originally written when assistive technology relied on browser parsing being deterministic. Modern browsers and assistive tech have converged on the HTML5 parser, which is forgiving of most legacy violations. WCAG 2.2 marked this criterion as obsolete and removed it from new conformance baselines.
For sites still targeting WCAG 2.1 conformance, the auditable failures that remain meaningful are: duplicate IDs (which break `for=` and `aria-labelledby` references) and duplicate attributes on a single element.
Common Shopify failure
Theme that renders `<h1 id="hero">` twice on the same page (once in the header, once in a section). Newsletter form that has two `<input id="email">` because both desktop and mobile partials are visible.
How to fix it
AccessComply scans for duplicate IDs across the rendered DOM and rewrites the second occurrence to use a unique suffix (`-mobile`, `-desktop`).
Primary source: W3C — WCAG 2.0 Understanding 4.1.1