This was my reason for foraying into Tcl too. Till then I had used Java's AWT and Swing; but prototyping with Tcl/Tk was way faster and intuitive. Mid-2000s. I ended up liking the language - idiosyncratic as it was. Partly the reason I am curious about and planning to read Ousterhout's (creator of Tcl) book "A Philosophy of software Design" soon.

It’s a great book (only one reference to Tcl that I recall though). Also Perl/Tk was the first GUI toolkit I used (followed by Glade).

The event handler based vector graphics in Tk were unmatched until HTML 5 and the canvas. And then there was the easy and fantastically well documented C extensibility...

Tagging system of Tk's canvas is unmatched in HTML. Pity they (HTML designers) never learn.

Basically, in Tk, you can add a tag to an element of your drawing in canvas and assign mouse (or keyboard) event handlers to change colors or other attributes on entry and exit of mouse cursor.

I made a OO/relational database schema visualization in Tk, it was breese to navigate - once you hover over a relation, you can see classes highlighted; follow highlighted relation to get to them. And once you highlight class of objects you can see to which relations it belongs.

It tooks two foreach loops to get that behavior. As far as I know, HTML5 canvas does not support that out of the box.

HTML canvas indeed doesn’t support hit detection – although SVG does, and you can embed SVG in HTML.

I think it still is. A very easy to use, quite well working cross-platform solution. Since 8.6, the UI has picked up a much more nativel look and feel.