The text isn't greyed out, and the cursor is wrong too. <textbox type="number" disabled="true" value="20"/>
Created attachment 473149 [details] [diff] [review] patch? This seems to fix it, but I don't really know if there's a better way to do this, or whether this is complete given the other styling in that file.
Why does this only affect Mac?
Is this a regression?
I think you just need to remove the color and cursor properties from the .numberbox-input-box rule in numberbox.css. The correct color is already set on the textbox and just needs to be inherited down properly. For the non-disabled case, cursor:text is set by forms.css on the html:input and doesn't need to be set on the .numberbox-input-box.
Comment on attachment 473149 [details] [diff] [review] patch? I have no idea if this ever worked, or why it only affects Mac. Sounds like Markus' suggestion is better, though.
Created attachment 475554 [details] [diff] [review] v1 Can you test this on Linux?
Works for me on Linux - cursor is "text" when enabled, normal when disabled.