If you're gonna use some random CDN, you should use the integrity attribute to confirm the resource doesn't unexpectedly change.
I'd encourage users to generate their own CSS bundle, so you only pull in whatever styles you need. Myth looks to be unmaintained. You can achieve the same thing with postcss and cssnext [0].
The one time I tried postcss, I was pretty let down by how poorly plugins would interact with eachother. There's definitely a huge benefit to the less/sass model where e.g. variables are a first class concept that everything can appropriately make use of. In the first day of use I hit all sorts of edge cases that couldn't be solved with the sort of pipeline postcss does.
I don't think CSSnext is actively maintained either :/
Also, related to CSSnext — isn't the @apply spec idea dead? Does that make CSSnext “yesterday's vision of tomorrow's CSS” but not what we think the future of CSS will look like now?
I'd encourage users to generate their own CSS bundle, so you only pull in whatever styles you need. Myth looks to be unmaintained. You can achieve the same thing with postcss and cssnext [0].
[0] http://cssnext.io