Firefox support

**Status:** :warning: Still testing, not ready for merge. :warning: 

Changed from having ```html, body``` hidden and faded in on load to just ```body``` ([here](https://github.com/l1f7/l1f7.github.io/blob/bdd57b1f411c557c7507006c0275e81e8fe6c21a/assets/js/main.js#L555)). For some reason, Firefox doesn't allow or like having the ```html``` element faded. Also changed the relevant header CSS.

Don't think this should change anything, as it still hides all content visible to the user.