When I am composing an item for fortran-lang I often see words with a wiggly red underline warning me that they may be spelt wrongly. But I was surprised to see allocatable underlined. Indeed the word is not in the Oxford English Dictionary, but it is in Webster’s, and it has been a technical term in Fortran for decades. Curiously, the word allocable is in OED meaning the same thing but it is also red-underlined.
In Firefox, I can add the red underlined words in the dictionary used by the browser (with a right click on the word).