Add border-width: 1px for horizontal rules

[AlexVipond]

Fixes #979

[adamwathan]

Thanks @AlexVipond!

[alexdilley]

Was the intention to make <hr>s 1px in height? Because a 1px border on all sides makes it 2px tall.

[brunolemos]

Should probably be border-bottom-width: 1px;.

[adamwathan]

Yep agree, will fix this for the next patch release. Sorry that it'll be another breaking-ish patch 😩

[AlexVipond]

Was the intention to make <hr>s 1px in height? Because a 1px border on all sides makes it 2px tall.

My intention was just to find something that looked like a good default—I was designing in the browser, added the border class, and didn't give much thought to the actual pixel height of the element. Although if you put me on the spot, I probably would've have guessed they were 1px tall 😅

Personally I don't have a strong preference between a 1px or 2px tall default border. Maybe it's time for another @adamwathan twitter poll?