[sampl]

Deeply nested CSS selectors in the source (like the ones in your MixPanel examples) are often a sign of CSS preprocessors in use. Using a tool like LESS or Stylus, MixPanel is probably including mixins in their styles that allow them the same modular flexibility you describe without cluttering the HTML with confusing, non-semantic classes like "mrm".

Edit: your site's visual design is awesome, btw

[mcmillion]

This * 1000.

Just because the output isn't 100% optimal doesn't mean the input isn't sane, maintainable, or clean.

[nobodysfool]

The same could be said of facebook. MRM means Margin Right Medium (it may be that originally it was "Margin-Right-Medium" but the compressor made the substitution to "MRM").

[ultimatedelman]

true, however, remember that the output must be sane too since you're putting it in the HTML