Custom tooltips seem not to work

[hansmbakker]

See https://visjs.github.io/vis-timeline/examples/timeline/items/tooltipTemplate.html:

It should set a color on the tooltip, but the tooltip doesn't change its appearance.

Same for https://visjs.github.io/vis-timeline/examples/timeline/items/tooltip.html:
Item 3 should have a red tooltip, but it doesn't work.

In my own code I'm trying to use a template but there, too, the styling and classes seem to be stripped from the tooltip HTML.

[hansmbakker]

This is related to #1908

[LemarinelNet]

I think I found the issue, see this comment : #840 (comment)