Rare words highlighter

What is this?

This tool highlights uncommon and mispeled words as red.
Common words are light grey: the, about, find, last, copyright.
Less common words are dark grey: TCO, Lumix, substring, phenotypes, conspiracies.

Why did I make it?

I've been using Google search as a spell checker for years. When I search for "colocual" it corrects the word for me: "Showing results for colloquial". [Todo: make an example with a sentence fragment]

It's a lot of hassle to open a new browser tab and copy/paste sentence fragments one by one. It could be better.

How does this thing work?
It uses Google's database of 50k most common words sorted by usage frequency. It has proper nouns (UK, Microsoft, Bob), slang (todo: add an example), and popular lexemes (not just "go" but also "goes", "went", "gone", "going", etc.).

Currently, it only checks words one by one. However, Google has an n-gram database that, I think, can be used to make a spell checker that would spot errors like "I has" or "I just had a lunch.".