refactor: further optimization suggestions by webhint & lighthouse

[mfranzke]

I am using the Pattern Lab Node website and Pattern Lab documentation website hosted on https://patternlab-handlebars-preview.netlify.app/ (PL frame as well as styleguide.html) and https://patternlab.io/

On top of the most of the previously even already included accessibility reportings by webhint, there are still some optimization possibilities left:

• https://webhint.io/scanner/334bfdb9-b5a3-41a0-8819-d4ba0bedb830 (webhint seems to have discontinued their online reports)
• https://webhint.io/scanner/1f234578-eaa0-4cd5-8592-d77c0f69a7d5 (webhint seems to have discontinued their online reports)