http://jsfiddle.net/addyosmani/bkd4S/

This doesn't seem that hard to fix... http://jsfiddle.net/gnarf/bkd4S/1/

Although making a cssHook for this is probably overkill, adding "normal" to the "convert to zero" list in https://github.com/jquery/jquery/commit/85d9343271da85fc945bf37a604873eaf247a3a7/#L0R344 might be the better solution...

might be worth it to create a map of properties to zeroable values. Blindly converting "normal" to zero would produce unexpected results for font-weight (which has "normal" interpreted as 400, not 0: http://www.w3schools.com/css/pr_font_weight.asp) and other css attributes that have default values of "normal" that don't translate to 0.

here's a good list of css properties with initial values, just in case we want to make a map of properties that have text values that can be converted to float values. http://www.w3.org/TR/CSS21/propidx.html

from that list, it seems that there are really only a few properties that have default "normal" values that make sense to animate. Here's the list, with both what the "normal" value means, and the corresponding value when you get the computed value instead of the style value:

font-weight: 400 computed: unclear from the spec
letter-spacing: 0 (unless in a justified text block) computed: normal
word-spacing: 0 computed: 0
line-height: depending on vendor 1.0-1.2 (which is a multiplier of the font size of the text this applies to) computed: normal

Seems that despite the caveats, we would have to make a map. Browser functionality doesn't seems to return useful values for neither the style property nor the computed value.

could be something like:

var cssNormalTransform = {
 letterSpacing : 0,
 fontWeight: 400,
 wordSpacing: 0,
 lineHeight : 1
};
var parsed, r = jQuery.css( this.elem, this.prop );
if( r === "normal" && cssNormalTransform[this.prop] ){
 return cssNormalTransform[this.prop];
}
return isNaN( parsed = parseFloat( r ) ) ? !r || r === "auto" ? 0 : r : parsed;

https://github.com/jquery/jquery/pull/420

Almost a year since this ticket has been opened, any updates? We've been stuck with jQuery 1.4.4 because of this regression issue.

@anonymous: You know you can upgrade and define a custom hook for letterSpacing, right? http://jsfiddle.net/gnarf/bkd4S/1/

https://github.com/jquery/jquery/pull/797

Less letterSpacing .animate() fail in IE. Fixes #8627

Changeset: b9b87d53c681a8337cdbdbe81f8f4e577e5ec277

See also #14792