Use Autoprefixer instead of prefixing mixins

[PixievoltNo1]

Currently, when using CSS properties or at-rules that need vendor prefixes, this project uses Sass mixins that compile to all possible variants. This has drawbacks:

• The Sass source is harder to write, as it's difficult to remember which properties/at-rules have a mixin available.
• The compiled CSS is harder for developers making consuming applications to read, as it's bloated with many unnecessary variants. For example, no Microsoft browser has ever needed -ms- for any animation-related property or at-rule, yet vex provides every @keyframes a matching @-ms-keyframes rule.

IMO, a much better approach is to use Autoprefixer, which analyzes the CSS after Sass has done its work and adds only the prefixes needed according to the specified browserslist. I'm willing to do the work to migrate this project to Autoprefixer and submit a PR; I'd just like some indication that the work will be welcome.

This would entail a few things:

• Autoprefixer, PostCSS, and anything else necessary would become devDependencies of this project.
• vex's supported browsers would be formally specified in package.json. I plan on copying the versions over from the Browser Support section of the home page.

[bbatliner]

I've used similar tools in other projects, big fan. When I rewrote vex last year I focused on the JS because that was my expertise. An upgrade of the CSS side would be greatly appreciated! Thanks so much pikadude. Let me know how I can help.

[PixievoltNo1]

Thanks for the positive response; I'll gladly get that done!

I have a question I hope you can answer: Is pie-clearfix (defined in _mixins.sass and used in vex-theme-*.sass) still needed for IE 9+? If it turns out it isn't, we can drop the entire _mixins.sass file.