Squint автор Hyper Boar

Press alt+1, alt+2, alt+3 or alt+4 to turn web page into a grayscale mush.
Press Esc to bring page back to its original look.

Extension is intended for artists and designers who want to easily check grayscale structure of an images/videos on the page or the page as a whole without using any external tools.

alt+1 - standard sRGB grayscale conversion, aka just grayscale;
alt+2 - squint LVL-1, blurs everything a bit, slightly bumps contrast, reduces details;
alt+3 - squint LVL-2, blurs more, bumps contrast more, further reduces details;
alt+4 - squint LVL-9999, blurs hard, contrasts hard, splits page into "hottest" and "coldest" grayscale areas.

Known issues:
- Sometimes some elements might visually disappear, but they are still there. To turn page grayscale I use an SVG filter, which in turn might mess up the rendering of some elements on the page.

Source code on GutHub