The MediaWiki software, which drives Wikipedia, allows the use of a subset of HTML5elements, or tags and their attributes, for presentation formatting. But most HTML can be included by using equivalent wiki markup or templates; these are generally preferred within articles, as they are sometimes simpler for most editors and less intrusive in the editing window; but Wikipedia's Manual of Style recommends their use in certain cases. And HTML is useful outside of articles, for example for formatting within templates. For help with Cascading Style Sheet use within Wikipedia see Help:Cascading Style Sheets.
Some tags look like HTML, but are actually MediaWiki parser and extension tags, and so are really wiki markup. HTML in pages can be checked for HTML5 compliance by using validation. Some elements and attributes are supported by MediaWiki and browsers, but have been deprecated by HTML5, and therefore should no longer be used.