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As you’ve already learned, HTML5 loosens some of the rules. That’s because the creators of HTML5 wanted the language to more closely reflect web browser reality—in other words, they wanted to narrow the gap between “web pages that work” and “web pages that are considered valid, according to the standard.” In the next section, you’ll take a closer look at how the rules have changed.
Of course there are still plenty of obsolete practices that browsers support but that the HTML5 standard strictly discourages. For help catching these, you’ll need an HTML5 validator (HTML5 Validation).