[FEATURE] HTMLLabel to support stylesheets and css_classes

[carve11]

Problem description

The HTMLLabel or Label annotations are very useful renderers for text since they accept screen units. One issue for me is the lack of css style options beyond the current text, border and background arguments. I often find that I would like to style something but not able to. Eg it could be padding, use the font-family defined in html body css, font-weight. I have previously asked about this on discourse.

Based on the following versions:
Python version : 3.11.7 (main, Jan 1 2024, 01:39:59) [Clang 14.0.0 (clang-1400.0.29.202)]
IPython version : (not installed)
Tornado version : 6.4
Bokeh version : 3.3.3

Feature description

Other widgets support the arguments css_classes and stylesheets and, if I have understood correctly, HTMLLabel is a div element.

Potential alternatives

Have not considered any alternatives.

Additional information

No response

[mattpap]

HTMLLabel was designed to be an HTML equivalent to canvas Label annotation, thus it strives to keep the same API, behavior and appearance. In fact, HTMLLabel used to be implemented as Label(render_mode="css"). However, the utility of HTMLLabel defined like this is questionable. We are currently in the process of adding "floating" UI elements to bokeh (PR #13538), so it will be possible to add HTML panels to plots/canvas and any renderers (e.g. attach HTML to a box annotation) in screen, data and symbolic coordinates. These HTML panels support css_classes, stylesheets and more.

[carve11]

@mattpap Cool, thanks for the update. Looking forward to the floating UI elements.

[mattpap]

PR #13828 adds support for all styling properties of UIElement to all DOM based renderers on the canvas.