Support for Custom HTML Markers

[buzzswarbrick]

https://www.mapbox.com/blog/custom-markers-mapboxgl/

In the release Mapbox GL JS (v0.21.0) they have support to add markers, and with the example there looks to be much less delay in positioning in the markers VS adding a custom HTML and manually mercating using the available viewport event.

It would be great to see if there is a performance in rendering the html element is improved.

[cammanderson]

We will be actioning this shortly