The semantic argument is what you get when graphic designers and web monkeys try to articulate separation of concerns. The only way they know to partition a system is at the CSS-HTML boundary, because that's what they were spoon fed. This mindset forces them them to viciously make all HTML human readable: it is their only tool for managing system complexity. They take things like the W3C's statement that tables ought to be for tabular data only as Revealed Truth, to be enforced without question. (Seriously. A lot of them consider violating these principles to be a career-ending move, and adjust their hiring policy accordingly.)

The engineers think that separation of concerns is something the system designer imposes. Does your fancy button use a squirrelly pile of implementation weirdness? No problem, just create a templating mini-language with a simple syntax for expressing a button; hive off the squirrelly stuff into its own module. In their minds, making a button with a 9-cell table is Baby's First Custom Widget, at least compared to frameworks like GTK+.

Naturally there is ... friction between these two schools.