[KevinH]

Okay, I tried again and I think I had inspector examining the wrong line.

I see the "user agent stylesheet" listed with the conflicting styles (font size and font weight) with a line through them in the "user agent stylesheet".

That is because the stylesheet with your code is overruling the "user agent" stylesheet. To illustrate, change your css to font-size: 250% and you will see the resulting Preview image grow considerably in size.

Your css stylsheet is taking precedence not the built in default styling for the "h1" tag they call the "user agent".

Are you sure you remembered to link your stylesheet with the xhtml file?