About This HTML Issue
The <icon> element does not exist in HTML. No version of the HTML specification defines it, and browsers do not recognize it.
This error appears when markup includes <icon> as if it were a standard HTML element. It is not. Browsers will treat it as an unknown inline element with no default behavior or semantics. To display icons, use an <img> element, an <svg> element, or a <span> with CSS-applied background images or icon fonts.
If the intent is to define a favicon (the small icon shown in browser tabs), the correct approach is a <link> element inside <head> with rel="icon".
HTML examples
Invalid: using the <icon> element
<head>
<title>My page</title>
<icon src="favicon.png"></icon>
</head>
Valid: using a <link> element for a favicon
<head>
<title>My page</title>
<link rel="icon" href="favicon.png" type="image/png">
</head>
Valid: displaying an icon inline with <img>
<p>
<img src="star.svg" alt="Star" width="16" height="16"> Favorite
</p>
Valid: displaying an icon inline with <span> and CSS
<p>
<span class="icon icon-star" aria-hidden="true"></span> Favorite
</p>
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