Need to specify how map-a links are accessible to screenreaders

[prushforth]

Currently the text content of a map-a tag is the geometry value for a feature, or some part of it. This is great for sighted users, but will be inaccessible to screenreader users.

We need a way to allow human-readable text to provide the link text, probably via an attribute. The HTML area element uses the alt attribute.

[Malvoz]

The global HTML attribute title and ARIA aria-label should suffice, no need to specify alt for links.

Linking #181.