Implement postcss-resemble-image for the header

[ben-eb]

Would be good to have this in case of a slow internet connection, it's a bit more interesting than a solid colour;

https://github.com/ben-eb/postcss-resemble-image

Original article: http://csswizardry.com/2016/10/improving-perceived-performance-with-multiple-background-images/

[ai]

More PostCSS plugins in PostCSS website! :)

/cc @marcustisater

[marcustisater]

Yeah, I'm sorry😢 I have been super busy with planning WordCamp, I promise to look it through and sort of some stuff by the end of the month. We need to get the basic pages up as well so we can start with #26

[marcustisater]

I will have a look at this tomorrow and also update pheonomic.