Not denying there was plenty of innovation in the layout manager space. But it's very possible that the standards committees for browsers weren't looking to those for inspiration at the time, especially before Gmail showed what was possible. Of course they might have done everything right, but just noting that hindsight is 20/20.
These kinds of layouts were incredibly common on the web by late 90s already. I remember wondering back then why all the desktop UI frameworks (it wasn't just Tcl!) have such flexible and powerful layouts, but HTML/CSS does not, even though it would obviously help solve a very common need.
