That's a great idea to make this browser-based! I like how clean and simple this is, with a super easy selection of characters. That's really great!
I've recently played with some ASCII (and variants) tools like lvllvl.com (my favorite). I quite enjoyed the possibility to use specific fonts or tilesets and to set the background and foreground color per character.
Although ASCII is too limited for me, I'm thinking that a box (rectangle or filled) tool, as well as a line tool could be interesting to add. Just an idea!
Here's something I've done in lvllvl.com with a specific vector set, layers and a bit of Photoshop for the texture. Just to show you what I'm referring too when I'm speaking of custom fonts.
This by the way based on this open font: https://www.kreativekorp.com/software/fonts/ksquare.shtml with the added ability to rotate tiles.
Here's another use of the same tool for the background (not characters) with a specific 8x8 font
I just wanted to share this interesting use of text art with you :) Tools like Playscii and Rexpaint also go into this direction, although I didn't enjoy their MS-DOS like interface much 😅