Rachel Andrew reminds us that the power new CSS layout methods gives us could be used to form new anti-patterns:
With this power comes great responsibility. For just as it will be possible for a developer to start out with a beautifully semantic, well structured document and use Grid and Flexbox to meet the design requirements, it will be possible for them to stop caring about the document structure at all. Worse, I believe there will be a strong temptation, especially with Grid, to flatten out document structure in order that all elements become a child of the element with the Grid declared. Making layout simple, but at what cost?