This bug doesn't happen for IE and Opera. But it shows in FF and Chrome.

This isn't an animate() bug but has to do with css().

test case

(Recently saw this) similar/related report #5526. Sometimes there seem to be problems with short-hand properties.

Instead of the workaround on stackoverflow you could also include the marginpadding.js from Brandon Aarons jQuery-cssHooks. This also fixes the problem

Your test case with jQuery-cssHooks

I have updated the .animate documentation to match the .css documentation which states that shorthand properties are not supported.

@snover

Thanks for updating the documentation. But the copy-paste from the css() looks rather unfortunate. Maybe you should replace

For example, if you want to retrieve the rendered margin, use: $(elem).css('marginTop') and $(elem).css('marginRight'), and so on.

with

For example, if you want to animate the margin property, use

$(elem).animate({
    'margin-top' : 10,
    'margin-right' : 10,
    'margin-bottom' : 10,
    'margin-left' : 10
}, 1000);