Just noticed that issue on Google Chrome 5.0.342.9 beta - Debian Lenny (2.6.26-2-686 GNU/Linux).

Resetting milestone to future.

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Issue still exists: http://jsfiddle.net/X43SS/

And still on Linux Chrome 6.0.472.63

http://jsfiddle.net/U2427/1/

Seems like any subsequent call to fadeTo() (after the initial one) on given element starts with opacity at 0.

This bug is caused by the fact that animate() assumes px unit for all values where unit is not given. Linux Chrome seems to be offended by that in case of opacity. After adding special case for opacity everything works properly.

Patch for jQuery 1.4.4 that does just that:

6490c6490
<                                               e.custom( start, end, name != "opacity" ? unit : "" );
---
>                                               e.custom( start, end, unit );

and same patch but for src/effects.js file in jQuery GIT:

206c206                                                                                                                                                                                                                                                                        
<                                               e.custom( start, end, unit );                                                                                                                                                                                                  
---                                                                                                                                                                                                                                                                            
>                                               e.custom( start, end, name != "opacity" ? unit : "" );                                                                                                                                                                         

I also forked jQuery at github applied this patch and made a Pull request. https://github.com/KamilSzot/jquery/commit/53b7d87f056fb03dc39a33513a4ac9106c36a28a

I made a mistake during testing and those patches do not fix the problem. Also assuming px unit does no harm as there is special case for opacity in jQuery.fx.step

I think I've found the real cause. It's bug in Chrome. I've filed the issue there: http://code.google.com/p/chromium/issues/detail?id=69180

You can work around the bug by replacing line:

		var r = parseFloat( jQuery.css( this.elem, this.prop ) );

with:

		var r = parseFloat( jQuery.css( this.elem, this.prop ).replace(",", ".") );

Below is much more elegant workaround, that I can use with standard minified release version of jQuery.

Just put it your code before using opacity animation:

			var standardOpacityHook = jQuery.cssHooks.opacity.get
			jQuery.cssHooks.opacity.get = function( elem, computed ) {
				return standardOpacityHook(elem, computed).replace(",", ".");
			}