Emacs and Vim have rectangle-editing commands, these are essential for reasonably convenient editing.
They're so useful, everyone should know them anyway: Need a column of '//' rather than '/* <long_comment_block> */', or vice-versa?. One click-drag and 4-5 keystrokes will do it.
Comments
Ideally you need a block based editor. Kate is the only modern editor that I’ve used that I know has a block mode. Any others?
Now the question is how do you get a block based editor on a webpage?
I don't think I expect people to edit the diagrams on webpages, rather than locally using their favorite text editor.
Emacs and Vim have rectangle-editing commands, these are essential for reasonably convenient editing.
They're so useful, everyone should know them anyway: Need a column of '//' rather than '/* <long_comment_block> */', or vice-versa?. One click-drag and 4-5 keystrokes will do it.
notepad++ has block editing
Depending on how you define "modern", vim (and I assume neo-vim) support block-mode.
You can move rectangular blocks around in VSCode
mcedit in mc file manager has block mode selection (Shift-F3).