The issue is not about vertical-align
on <td>
but on <img />
Quirks mode triggers a behaviour explained here:
Vertical alignment of an image is under certain conditions to the bottom of the enclosing box, not to the baseline of text. This happens when the image is the only content within an element, typically a table cell. This means that e.g. an image in a table cell is by default at the bottom of the cell in Quirks Mode (which is often what the author wants), whereas in Standards Mode there is a few pixels spacing below the image (unless one sets e.g. vertical-align: bottom for the img element).
The space you see in standard mode below the image is actually the space between the <td>
's box baseline and bottom (cf. http://www.w3.org/TR/CSS2/visudet.html#leading).
When vertical-align
is bottom
on <img />
its box' bottom is aligned with <td>
's box bottom, so there is no space anymore.