It's a little complicated and (I'd argue) mildly broken[1], but roughly, 'in', 'cm', and 'pt' all refer to real-world distance measurements; 'em', 'en', and 'ex' refer to sizes relative to the font size; 'px' is a modified real-world measure that scales with distance[2]; and there is nothing at all that refers to actual, honest-to-god pixels. Scaling 'px' during a zoom is no more standards-violating than scaling 'in' etc. during a zoom.

[1]Because 'px' doesn't refer to pixels.

[2]It refers to a unit that scales according to display resolution and usual distance from eye; essentially it's a measure of the angle subtended on your retina. This is useful because (in theory) 1px will look "the same size" (and be about as legible) on a mobile phone, on a laptop, on a desktop, or on a projector, whereas 1in or 1pt(=1/72in) will appear much smaller if further away. What 1px is not, however, is a "pixel".

Now comes the question: if 'px' isn't it, is there any unit that corresponds 1:1 with a bitmapped image?

At default zoom level, 1 px should be 1 pixel except on "retina" displays where it is 2 pixels or printers where it is 3-5 dots. http://webkit.org/blog/55/high-dpi-web-sites/ http://www.w3.org/TR/CSS21/syndata.html#length-units

Yes, px. Bitmaps should be scaled from device pixels to CSS pixels.

Would browser zoom be under the purview of the CSS standard?

Do you mean zoom, or changing font size? Because zoom scales everything, images included. Scaling affects only text, and ideally, things that are sized relative to the text (but not pixels).