Trim trailing spaces and use 2 spaces consistently for indentation.
Fix validation errors.

All validation errors are fixed except for the `http-equiv="X-UA-Compatible"` meta tag which is known to not be HTML5 valid but is needed in order to force usage of the latest IE.
Remove stray space after `alt=`.
index.html: remove unneeded class.

It's inherited from `ul`.
Fix links.

* use the https links to GitHub for consistency
* fix kramdown links target
Use the same dimensions as in the css for .avatar.