The third line should work for what you want because of strange layout
anatomy quirks, basically - .datetime is actually a span (which is a type
of inline element) and not a div (which is a type of block element). Span
elements won't stretch out to nice background blocks because they're meant
to be used within stuff like a paragraph without disrupting flow. Block
elements like divs will stretch out and disrupt flow if you use them in
the middle of something.
So .comment .header is a div, and putting a background color on it colors
both behind the subject and the date/time. HOWEVER these layouts have a
color behind the subject (which is an h3, which is also a block level
element), and that colors behind the whole subject block and sits on top of
the color for the header block, so the header block color only shows behind
date/time.
no subject
The third line should work for what you want because of strange layout anatomy quirks, basically - .datetime is actually a span (which is a type of inline element) and not a div (which is a type of block element). Span elements won't stretch out to nice background blocks because they're meant to be used within stuff like a paragraph without disrupting flow. Block elements like divs will stretch out and disrupt flow if you use them in the middle of something.
So .comment .header is a div, and putting a background color on it colors both behind the subject and the date/time. HOWEVER these layouts have a color behind the subject (which is an h3, which is also a block level element), and that colors behind the whole subject block and sits on top of the color for the header block, so the header block color only shows behind date/time.