improve accessibility

[ChristianSch]

TODO

• tabs are not focusable by keyboard (but markers in the map are) - focus tabs first then tab content
• when zooming in (cmd + +) the layout get's messed up and the footer hides the content
• missing alt-tags (<img>)
• no skip-link

Ressources

• Easy Checks - A First Review of Web Accessibility
• Accessible Rich Internet Applications
• http://whatsock.com/tsg/

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