Custom text styling such as box-shadow or background-color

You can change color, font size, and font family for a text of a scatter point.
See example

Also you can apply trivial tags inline such as text=["<b>Text G</b>", "<i>Text H</i>"]

Are there any tricks/hacks to apply custom CSS to text such as '<div style="box-shadow: 0 0 5px brown;">Scatter point</div>'?

No annotations, no dash.

Answering my own question: it turned out the text on the plot is an SVG text. Use text-shadow instead.