You can use an HTML5 Validator tool to verify how well a website supports HTML5. There are many validation tools available online, but they should all conform to the most recent HTML5 specifications. One such tool produced by the W3C is called the W3C Markup Validation Service, at

The W3C Markup Validation Service is fairly thorough. It enables you to verify a website against non-English character-encoding types and older markup languages, such as HTML versions 4.01, 3.2, and 2.0, various XHTML standards, MathML, Scalable Vector Graphics (SVG), and Synchronized Multimedia Integration Language (SMIL).

When you connect, you must submit the URL of the site that you want to validate. ...

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