Same as how browsers compete with new HTML tags and such - CommonMark + few extensions could be standardized) with others left to the browsers to decide.

HTML inside markdown is anyway legal as per the standard, so anyone could use <table> for anything complicated anyway.

Such an implementation would also lead to users having more control and say in how the web pages look (similar to Gemini), by letting you read most content across most websites in fonts, styles, and colors of your choosing. This aids a lot in accessibility as well.