Formatter/Validator for HTML

[exoego]

Writing valid HTML and format are sometimes forgetten.
Then we need to spend energy to fix them like doing so in #151.

I think automatic and uniform way of validating/format saves our energy for sustaining docs.
Let's consider to introduce something like https://prettier.io/ into CI.

[y0hami]

Agreed, we might want to look at alternatives to prettier because it will complain about probably 90% of the codebase and that will be a lot of work to change.