Can your monitor display a wide colour gamut? If you see a uniform red square, your monitor cannot display a wide gamut of colours.
If you see the W letter inside the square, your monitor is wide gamut.

How far from sRGB is your display color gamut? The more difference you see inside the squares, the further your display color gamut is from sRGB standard.

ProPhoto tagged image

sRGB tagged image

How does your browser interpret untagged images? All untagged images and page elements should be considered sRGB by the web browser, according to The World Wide Web Consortium (W3C). If your browser respects this standard, you should see perfectly uniform colour squares above.

sRGB untagged

sRGB tagged image

How does your browser interpret page elements? All untagged images and page elements should be considered sRGB by the web browser, according to The World Wide Web Consortium (W3C). If your browser respects this standard, you should see perfectly uniform colour squares above.

Untagged CSS element

sRGB image