HTML Guide

Elements that have the xml:lang attribute also need a matching lang attribute. In HTML5 documents, using just the lang attribute is enough.

Check the HTMLImageElement.srcset guide to learn about the correct usage of the srcset and sizes attributes.

Attribute values for HTML elements must be enclosed within quotes. Check for attributes whose values are missing quotes.

Example:

<!-- These are invalid -->
<div class=important></div>
<div class=important"></div>

<!-- This is valid -->
<div class="important"></div>