> It breaks the separation between structure and design

I have a problem with this argument because CSS _itself_ breaks that separation.

`border: 1px solid red;` is design, but `display: flex;` is structure.

In my opinion, it makes sense for display properties to live in the HTML because they are by their very nature very tighly coupled to the markup.

Though I appreciate that making this distinction creates a separate problem of your team having to exercise good judgement (and consistency) in which CSS belongs where.