[snickerbockers]

>we're still using squiggles decades later

What in this context is "we"? I can name plenty of programs that accept textual input yet don't even spellcheck at all. The green squiggles are hardly ever even seen outside of word processors, for which it is practically mandatory to implement a 1:1 copy of the latest MS Word UI.

Evidently my browser uses red squiggles for what it believes to be spelling mistake because it just highlighted "UI" in my previous paragraph. That there is a chronic pandemic of terrible spelling in just about every online form of communication proves that the red squiggles are not effective at grabbing anybody's attention.

>If there are no better ideas

Counterpoint: 4% of the population cannot distinguish between red and green so at the very least I can say with confidence that using colors (especially those two colors in combination) is not a sufficiently effective way to distinguish between poor spelling and poor grammar. If it were up to me I'd replace one or both of them with some simple geometric shape such as an ellipse or a rectangle that encloses the word or phrase which is incorrect. The shape could still be colored to make it stand out to people who can distinguish between colors as long as the shape for grammar and the shape for spelling can be distinguished from one another without knowing what color they are.

Another good alternative is blinking like that old HTML tag. I don't know if there are any common disabilities which prevent people from easily discerning motion on a computer monitor (other than disabilities that inevitably apply to anything on the monitor, such as severe myopia or blindness) but blinking cursors are apparently acceptable so probably not.