[hsivonen]

> in the case of offline where there are no http headers, the browser works out what encoding to use

For file: URLs, Firefox and, a bit less reliably, Chrome do work out that an HTML file is UTF-8. Safari does not.

> And since utf-8 is the only valid charset for HTML5

Correct.

> it should be the default.

It isn't. See https://hsivonen.fi/utf-8-detection/ for why.

Having <meta charset="UTF-8"> in the boilerplate is good advice.