The HTML style guide doesn't list what to do about language tagging

The HTML style guide [says to not use the `` tag](https://google.github.io/styleguide/htmlcssguide.html#Optional_Tags)

The [W3 site says to **always use the HTML tag**](https://www.w3.org/International/questions/qa-http-and-lang) because you need to specify the language as in

The google HTML guide does not say how to deal with language.

Setting the language is often important because browsers will often guess wrong and offer to translate pages for which no translation should have been offered.

Asking the server to send the correct http header seems right out. For one it requires the server to maintain some kind of metadata about each file. metadata that might get out of sync and metadata that might not be in control of the person creating the document.

It seems like google's style guide should say what to do here.