Default charset in HTML5

If an HTML5 document doesn't specify a charset, the validator flags an error like this:

"Error: The character encoding was not declared. Proceeding using windows-1252"

and then proceeds to flag further errors  ("Unmappable byte sequence") when it encounters utf-8 encodings of characters not in the windows-1252 set. Isn't utf-8 the default for HTML5?
RegardsNick Jacobs