This gives a consistent and faithful image. Doing it as plaintext would cause problems with reflowing, weird scrolling boxes, and generally might have you fighting with the browser text renderer.
Consistent? Faithful? I see a whole bunch of images in different styles with different fonts, colours, fairly consistently the wrong line-heights, and even severe JPEG artefacts in at least one case.
Doing it in plain text with `white-space: pre` (the default for the <pre> element) would work better.
Consistent over devices, not Consistent between the images. The point is that jpeg rendering is a whole lot more consistent between devices than html rendering.
