Tried a couple different greyscale extensions, but this is the one. Perfect.

It incorrect work.
RGB色数値の計算・処理が正しくない。
例
灰色→オレンジ色になる
明るい灰色→白になる
ページの下になにもない大きい空間が生まれる

「明度」ではなく「輝度」の数値をグレースケールに使ってください。

良い点：
「このページに有効」がある

Well this works as intended in all websites except for one case: whenever any video or image is fullscreened all I see is a black screen, the audio works fine,just cant see the video or image.

Very useful. One anomaly happens when viewing images by themselves, e.g. the URL ends in "jpg": although the image is successfully converted to grayscale, only the bottom half is visible. The top half of the image scrolls off the top of the window. Apart from that, it does its job well, and I like being able to choose which sites to use it on. Good work!

Works perfectly!