Charset

Looks for the Charset meta tag and makes sure it is UTF-8.

Almost every webpage will contain some text content. In order to display it properly, the web browser needs to know how that text is encoded. The most common coding (and the only valid encoding for HTML5 documents) is UTF-8. This encoding uses a variable number of bytes to represent any character, including emoji, mathematical symbols and languages that don't use Latin-script alphabets. It is backwards compatible with ASCII and is in use by the majority of webpages.

The Charset metatag must be located entirely within the first 1024 bytes of the document, so we recommend placing it at the very top of the <head> section of the HTML. This test will issue a warning if the encoding is missing or declared to be something other than UTF-8.

For more information on the Charset metatag, please read the documentation on MDN Web Docs and to learn more about UTF-8 you can start with this Wikipedia article.