This enhancement would solve a genuine need, and it would make 1:M labeling look really good.
See the following Blog post:
https://community.esri.com/blogs/richard_fairhurst/2015/02/07/creating-labels-with-related-table-data
There are no plans to use HTML and CSS in labels. The added complexity, performance, and security implications of essentially having a web page per label make this difficult. If you're just looking for table support, I'd recommend following this issue: https://community.esri.com/t5/arcgis-pro-ideas/create-tabular-labels-with-multiple-attribute/idi-p/927800
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