Explorer Mac, Mozilla and Safari treat font-weight: 600 and bold
exactly the same.
I found one other page that describes this problem, but it's in Dutch. No English pages yet found; if you know of them, please tell me.
In Explorer Windows and Opera there turns out to be a difference between
font-weight: 600 and font-weight: bold, bits of code that I
always thought were equivalent.
Here are test paragraphs. In Explorer Windows and Opera the font-weight: 600
paragraph has a slightly different font from the next two.
The quick brown fox jumped over the lazy dog. [font-weight: 600]
The quick brown fox jumped over the lazy dog. [font-weight: 700]
The quick brown fox jumped over the lazy dog. [font-weight: bold]
It seems as if these browsers stretch up their normal font a bit when encountering
font-weight: 600. The value 700 is the same as bold.
Conclusion: use font-weight: 700 or bold, and not 600.