I'm learning Motif just for fun with "Vol6a" and it's fairly easier than Xaw. When Motif was propietary, GNU users did an astounding work with LessTif, as programming in Xaw was far more level and prone to bugs because you had to keep more stuff set.

Also, Xaw3D was a good effort on mimicking Motif widgets, but it had the same exact API as Xaw. And, finally, in late 90's TCL/TK had a big boost under GNU distros because it was a quick way to plug an UI over some C software.

If you like Motif, you might like EMWM too. WIth the suggest addons and extras, it's like a mini-DWM from IRIX minus the launching dock; altough you don't need it, the toolbar it's more than enough.

A pity Mosaic it's propietary; adding auth support for NNTP it's something every C novice could do under 2 minutes with a printf (and I did). But there's some nice TCL/TK software which just requires tcl-tls and a plain TCL/TK install:

https://codeberg.org/luxferre/BFG