[accessibility] The slider control toggle is not keyboard operable

[Malvoz]

The <a> slider control is not keyboard operable. Additionally, it is announced as a "link" using a screen reader (missing role="button", though <button type="button"> is recommended).

Also see WAI-ARIA Authoring Practices recommended keyboard interactions for buttons.

[Malvoz]

Other potentially helpful resources:

• Leaflet Guideline: Test your maps
• Leaflet Plugin Authoring Guide: Content Accessibility