HTML / XML / JS, low-level validity (cross-cutting)MediumInferred

Missing DOCTYPE

Without a DOCTYPE, browsers fall back to quirks mode, rendering with legacy quirks that can break layout and behaviour. The standard HTML5 DOCTYPE avoids that.

What it is

No <!DOCTYPE html>.

Why it matters

Triggers quirks mode; rendering inconsistencies.

How to fix it

Add the HTML5 doctype.

How to find it on your site

  1. View source and check the first line is the HTML5 DOCTYPE.
  2. Add it if missing.
  3. Confirm the page renders in standards mode in DevTools.
  4. Re-test layout.

Cross-reference to ranking and citation factors

A missing DOCTYPE forces quirks mode, which can break layout and consistency. Declaring it keeps rendering standard.

Impact

Low-medium. Inferred.

Evidence

Standards-mode rendering supports correct display. Google Search Central, Understand JavaScript SEO Basics

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