🔍 Common HTML5 Validation Errors

Mismatched tags, duplicate ids, missing alt — why each one matters.

Here are a few mistakes that show up repeatedly when validating HTML5, and what each one actually breaks.

Mismatched tags

Opening a <div> and forgetting the closing </div> gets auto-corrected by the browser's own recovery rules, but often in a way that breaks the intended layout or nests later elements somewhere unexpected.

Duplicate id attributes

An id is supposed to be unique on the page. When several elements share the same id, document.getElementById() and the CSS #id selector both only pick the first match — the rest silently miss out on the intended style or script.

Missing image alt attributes

An <img> without alt gives a screen reader user no idea what the image shows, and leaves nothing to display if the image fails to load. It's a classic mistake that hurts both accessibility and SEO.

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