I think that HTML with CSS is easier to use and edit and it is probably not used so much because many people have a tendency to try to use the latest advertised. That does not mean that the latest is the best or even as effective and stable as the previous.
HTML with CSS is solid. I like it more than XML, even though each have their own situations of need. I do use XML (see
[X]). Nothing against it, but I use it for special needs. Common web pages with most of what is seen and done on the internet these days = HTML with CSS (and javascript and server side scripts) is best.
Rarely XML might be needed, but even then I tend to try to do the same with HTML and CSS and javascripts.
Further,
If you are using dynamic web pages
[X] and if you are using server side scripts
[X], meaning that your web pages are adaptable to the viewer and/or change due to some or other conditions, and meaning that your server writes the pages automatically, then I would very much suggest that you use HTML with CSS instead of XML. For the server-side website deployments you might like to use server-side java-scrips
[X].
In answer to "Why isn't pure XML+CSS more common?" I think that most people do not understand the power of "HTML+CSS+javascript+server applications" as compared to the limits of "XML+CSS". I think that many people are lazy or that many web developers have little idea of what they are doing.
But, they might just simply be far smarter than I am and I might not know what I am doing.
You, (not someone else) just you, should study and learn HTML, CSS, XML, javascript, server-side concepts. You do not have to learn them intensely, just learn about them.
Thank you for asking.
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