> Unless you know ahead of time the exact dimensions you will be displaying your book at you shouldn't use either.

This is incorrect. You can sure write LaTeX that is intricately dependent on the output dimensions. But you can just as easily write LaTeX that is independent of output dimensions.

Case in point is compiling LaTeX doc to HTML which you'd admit is easily resizable.

Case in point is also writing LaTeX docs for journals or publication where you can easily resize the document to match the publisher's style guide and dimensions by changing the documentclass.

Compiling latex to html means you have lahtml, not latex since it doesn't use the tex back end.

Using styles means that you use someone else's backed in page size settings and still need to recompile when you change to a new style.