I did find the bug ... I had not updated a SQL table (added a cell) on the preview site (not local) so a value couldn't be found.
I'm using Chrome. Using Inspect Element, Console, it showed nothing untoward.
I'm also no (good) JS programmer. I use it when I have to. For mundane things. What do you mean by "mark-up"? By JS lib, are you referring to something like jquery? Wouldn't know where to styart with that. As mush as today I have become to LOATHE CF, they do make it easy with cflayout, yet I have also found so many annoying things it does! Like you cannot put a JS function call within one; has to be outside it or in another template. Yuck.
Transferable skills?
And don't get me started on Builder. What a POS. Bot nothing else works. And it crashes by the hour.