michiel26,
if the validity of CSS is really important to you, removing this part of code is only right way. After this, if you insert an image larger than 490 px (in width) into post, in MSIE6 it will be cropped from right instead of resized (downsampled). Reason is simple - MSIE6 doesn`t recognize declaration max-width: 490px; like modern browsers (MSIE7, FF, Opera) do...
I.e., in MSIE6 will be visible left part of unresized image only - see this screenshots comparsion
Possible solution(s):
- you can switch from Visual to HTML editor and add width="490" to img-tag
- you can do it on "Insert/edit image" palette (card "Appearance")
- you can use some gallery-plugin (e.g. NextGEN Gallery) for images (but i`am not sure, if NextGEN have standard-compliant CSS, most likely not...)
- you can prepare your web-images with exact dimensions in your photo-editor, in this case you have full control over image quality (unlike automatic resizing in browser)
...etc...etc...
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(EN isn`t my mother tongue... but the message is clear, I hope... :-))