In Safari on my non-high-gamut Mac, the logo is clearly visible. (Edit: To be specific, on the default "Color LCD" profile I get 252,13,27 and 238,12,25 as native equivalents of the sRGB 255,0,0 and 241,0,0. Presumably there is some slight clamping around the edges of the profile, as well as some inherent precision loss if WebKit is performing the conversion on 24-bit colors, but it's not anywhere near as crazy as making 241,0,0 and 255,0,0 look the same.)
I didn't comment on the handling of that image. My comment related to the bit of the article I quoted, which states that WebKit treats images without a color profile as sRGB.
Then you're conflating two different transformations. Squashing 241,0,0 and 255,0,0 is the transformation from P3, as performed on that image. As I stated in the last bit of my comment, the transformation from sRGB to the display's native color space, as performed on images without a color profile, CSS colors, etc. [edit: and of course images tagged as sRGB!] is far more subtle.
https://webkit.org/blog-files/color-gamut/Webkit-logo-P3.png
does have a color profile. By the way, it also uses 15-bit color channels.
Here is the same image after pngcrush removed the color profile PNG chunks:
http://dl.dropbox.com/u/1237941/Webkit-logo-profile-removed....
In Safari on my non-high-gamut Mac, the logo is clearly visible. (Edit: To be specific, on the default "Color LCD" profile I get 252,13,27 and 238,12,25 as native equivalents of the sRGB 255,0,0 and 241,0,0. Presumably there is some slight clamping around the edges of the profile, as well as some inherent precision loss if WebKit is performing the conversion on 24-bit colors, but it's not anywhere near as crazy as making 241,0,0 and 255,0,0 look the same.)