Minify css with clean-css

[XhmikosR]

The reasons for this change are:

1. better compression
2. clean-css works around a couple of old IE issues like the one which needs a space after comma in filter properties (hence bootstrap-theme.min is bigger with clean-css)
3. clean-css is in active development and next thing is to improve selectors merging so compression should improve even more

For bootstrap.min.css

102.931 master.css
100.728 clean-css.css

/CC @mdo

[mdo]

Nice, sounds good to me. @cvrebert Feedback?

[cvrebert]

Might as well switch from RECESS to grunt-contrib-less then. It supports less v1.5 and clean-css-based minification.

[mdo]

Was thinking about that, too. Let's look into that for sure.

[XhmikosR]

I haven't used grunt-contrib-less, but if it supports both I guess it makes more sense to switch to that.

[mdo]

@XhmikosR @cvrebert The only problem is we'd lose some of the niceties that Recess provides:

• Proper alignment of prefixed properties
• Property order

Those are pretty huge to me, so I'm unsure about pushing forward on it. If that stuff is configurable, I'm super down. Otherwise might not work out. Anyway, enough diversion for this PR—let's talk about this more elsewhere if someone wants to start in on it.

[acmetech]

@mdo, these may help. https://github.com/csscomb/csscomb.js and https://github.com/csscomb/grunt-csscomb

Proper alignment of prefixed properties

https://github.com/csscomb/csscomb.js#vendor-prefix-align

I've never used the above, only the plugin for Sublime Text.

[mdo]

Merged into #11790.