Missing document language
This one is trivial to fix in most cases. Yet, it isn't in these situations:
- when working in big enterprisey projects.
- when working with a Single-Page-Application architecture. Angular, React and friends often forget about setting it, because the setting is in the static html and not in the dynamically generated markup.
- with authored content. Either autodetect the language or let the user specify it. I usually forget specifying the language on social media.
To set the document language, you append a lang-attribute to
your <html> tag. The value is a with the 2-letter language code
of the language you use in your document.
You can also add the lang attribute to just parts of the document.
This allows you to embed content in other languages to your page.