What strikes me about stories like this is how many invisible decisions shape the interfaces we can't imagine living without. The squiggle wasn't a product requirement or a committee decision — it was one person's intuition about how to surface information without interrupting flow.
The analog31 comment about "yellow squiggles for logic errors" is a genuinely interesting design problem. Linters do this in IDEs, but the reason it hasn't made it into general productivity software is that spell errors have a clear ground truth (the dictionary), while logic errors require understanding intent. The difficulty scales completely differently.