Blackrockrose, I did forget to mention the checkbox. But, if you had looked at the two screen prints I included in my original post as if it was a "spot the differences" puzzle, you could have noticed there was a box ticked in one image and not in the other!
TRex, to be honest I'm not sure when I decided to drop the upper case type for the big words. I've been working on this for quite a while and trying to juggle changes in different versions. I'm pretty sure the font hasn't changed: it's specified as "Verdana, sans-serif", but precisely which font is used is up to the browser and operating system. On the other hand, the font of the letters on the blue and yellow tiles is different. It's from Google Fonts and it's called "IBM Plex Sans Condensed". I chose it because it's quite narrow, even the M and W, so I could make the tiles tall and thin.
I think the longest word that can ever be played is 16 letters. By sheer coincidence it came up in yesterday's 7bm puzzle: antitotalitarian. There are thousands of longer words in our lexicon now, but they all have more than seven different letters. But to be on the safe side, it is possible to play more letters - if a 17th letter is played everything up there shrinks to make room for up to eight more letters.