Great point! Similarly, I sometimes use Emacs' excellent (and near-unique) electric-indent as a hint of syntax brokenness. "What do you mean this is getting indented at that lev-- oooh"
The downside with broken syntax highlighting (and electric-indent!) is when the editor's parser is insufficient, as is often the case with basic online editors, and breaks with legitimate constructs (Emacs with certain C macros). Then I can't trust the highlighter and also I have less-legible code.
I got so used to electric-indent and the immediate feedback it gives, that for a very long time it prevented me from even considering any other editors.
These days I rely on clangd driven autoindent (which is fast enough to do every line), but I still use emacs because it is so easy to tweak the interaction to clangd to work exactly as I prefer.
The downside with broken syntax highlighting (and electric-indent!) is when the editor's parser is insufficient, as is often the case with basic online editors, and breaks with legitimate constructs (Emacs with certain C macros). Then I can't trust the highlighter and also I have less-legible code.