Google prefers JSON-LD though[0]. Also I'm not keen on defining content with styling classes.
[0] https://www.searchenginejournal.com/google-structured-data-p...
[0] https://www.searchenginejournal.com/google-structured-data-p...
See https://html.spec.whatwg.org/multipage/dom.html#global-attri...
> authors are encouraged to use values that describe the nature of the content, rather than values that describe the desired presentation of the content.
Or could it be that the meaning here is that you should write descriptive classes such as 'alert-box' instead of 'red-box'.