Check web page for accessibility

[hannob]

Check web page with various accessibility tools and address issues.

Google PageSpeed Insights has some accessibility checks:
https://pagespeed.web.dev/

Furthermore:
https://www.accessibilitychecker.org/
https://wave.webaim.org/

[hannob]

Through the nlnet project funding, an accessibility report with improvement suggestions was created:
HAN Accessibility Report badkeys.pdf

[hannob]

Related commits where I forgot to tag this issue:
2d0cb06
6ad9554