What is the current state of sub-pixel accuracy in the major browsers?

Question

I'm working on a drawing application which requires high levels of accuracy, and I'm wondering which of the major browser platforms (including the HTML Canvas element, and Flash) give the best sub-pixel layout accuracy, both for drawn elements (rectangles in the Canvas or Flash, absolutely positioned DIVs in the browser), and for text.

There are a number of posts related to this, both on this site and others, (see list at bottom), but many are quite old, and none summarises the current situation.

My understanding is that Flash has native support for sub-pixel positioning, using twips to position objects to one twentieth of a pixel, and that when the TextLayoutFramework is used, this accuracy also extends to text. There is at least one report, however, that this doesn't work properly in Chrome. Can anyone confirm this?

My understanding of the situation in the browsers is that Firefox 14+ supports sub-pixel positioning for text and drawn elements, both in page layout and within the Canvas, but I haven't been able to ascertain how accurate this is.

I understand Chrome (as of v21) does not support sub-pixel positioning at all.

I understand IE9 doesn't support sub-pixel positioning, but it appears from the MS blog post linked below that IE10 will.

I don't know if there's any Mac/PC variance in this, and I don't know also if the accuracy of Flash varies between platforms and/or browsers.

I understand a summary question like this may provoke some debate, but I believe this is specific enough for people to provide useful answers, and hope that this thread can be a reference for the state of positioning accuracy up to now.

Some references:

http://blogs.msdn.com/b/ie/archive/2012/02/17/sub-pixel-rendering-and-the-css-object-model.aspx

Sub-pixel rendering in Chrome Canvas

http://johnblackburne.blogspot.co.uk/2011/11/twips.html

http://ejohn.org/blog/sub-pixel-problems-in-css/

Sub Pixel CSS positioning

https://productforums.google.com/forum/?fromgroups=#!topic/chrome/pRt3tiVIkSI

Answer 1

Currently, you can expect the best rounding and sub-pixel support to come from Mozilla with IE as the runner up. IE might end up being more fine tuned, but their release cycles are so long that Mozilla is likely to stay ahead of them.

As far as doing sub-pixel layout, you may be chasing a wisp, because the sub-pixel advantage improves anti-aliasing issues, not screen location accuracy. Your image will never be more accurate than 1 pixel from the true position, regardless of sub-pixel support.

The reason why some browsers don't zoom properly has nothing to do with sub-pixel support, it is because they are not remembering the exact position and rounding correctly. In other words, they are prematurely rounding the position and that causes the image to be mis-aligned.