Fix docs (#2482)

* Fix #2414, address part of #1687

Also cleared the CSS up to be a lot more readable instead of smooshed
into a single line.

* Redo the testing docs page

- Addresses another part of #1687
- Also, fix a few linter issues in the ospec binary

* Add note about third-party cookies, tweak a line

* Make the JSX comparison much more meaningful

And let the code speak for itself. Don't fuel the flame wars any more
than what they've already become. We should be *unopinionated*, and so
I've updated those docs to remove the existing opinion.

* Remove a bunch of outdated ES6 references

* Remove the CSS page

docs/releasing.md

@@ -20,6 +20,7 @@ $ git pull --rebase mithriljs next
 2. Determine patch level of the change
 3. Update information in `docs/change-log.md` to match reality of the new version being prepared for release
 4. Replace all existing references to `mithril@next` to `mithril` if moving from a release candidate to stable.
+    - Note: if making an initial release candidate, don't forget to move all the playground snippets to pull from `mithril@next`!
 5. Commit changes to `next`
 
 ```