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19 November 2012]

WAI R&D Symposia » Text Customization Home » Proceedings » This paper.

This paper is a contribution to the Text Customization for Readability Online Symposium. It was not developed by the W3C Web Accessibility Initiative (WAI) and does not necessarily represent the consensus view of W3C staff, participants, or members.

Accessibility issues due to sub-pixel rendering

  • Anthony Lee

I am grateful for the opportunity to submit a paper to the Text Customization for Readability Symposium in relation to issues which I have been experiencing when using the internet and web-based products due to sub-pixel rendering.

This paper is written from a user's perspective. It is not a technical research paper.


I typically use Windows PCs, laptops and notebooks (as opposed to Macs) to browse the internet.

I am colour-blind.

Unfortunately, I am unable to focus on text which is displayed on a flat screen when sub-pixel rendering software (such as Microsoft's ClearType) has been used to "smooth" the fonts. The text looks blurred to me (irrespective of adjustments). This causes visual disturbance and, typically, a bad headache as well.

As I understand it, sub-pixel rendering is designed to improve the edge contrast and readability of small fonts by using colours/shades at a sub-pixel level to trick the eye into seeing a "smooth" edge on a font instead of the "serrated/pixellated" edge which is actually there due to the square pixels on a flat screen.

However, it does not work for me. I see it for what it is: a pixelleted edge with a halo around it, which produces a blur. The best analogy of the blurring which I can give is the experience of watching a 3D film without the special glasses. Furthermore, the smaller the text, the greater the visual disturbance and the worse the headache.

Given the degree of confusion over colours/shades which comes with colour blindness, this blurring is consistent with my disability (although not all colour blind people are affected). In addition, from the desk- top research which I have undertaken (ie via Google search), it is apparent that a number of other people struggle with ClearType and complain of blurring. Examples of forum and blog sites which discuss issues with ClearType include:

Notwithstanding, the IT industry is pushing sub-pixel rendering. For example, Microsoft has integrated ClearType into its Windows 7 operating system as compared with the XP operating system where ClearType is an optional extra. Similarly, sub-pixel rendering is integrated into Apple's OSX operating system.

From a web accessibility perspective, the use of sub-pixel rendering affects me in a number of ways which are of increasing concern.

In this regard, it is also worth bearing in mind that until sub-pixel rendering started to take hold in 2007, I had always been able to use IT "out of the box" using default settings without the need to personalise in any way. Furthermore, I have been working for more than 20 years (ironically as an IT lawyer) and have seen huge changes in technology but without experiencing any issues with the graphical user interface and the display.

As such, I am somewhat perplexed and extremely concerned by the issues which now face me.

The principal issues are set out below.

1. Browser issues

I first started suffering from visual disturbance and headaches when browsing the internet with Internet Explorer after an upgrade from IE6 to IE7.

After research, it was established that IE7 had introduced ClearType but that it was possible to turn it off in html (in Advanced Options). This solved the problem for me. The option to turn off ClearType was carried through into IE8.

However, in IE9, it is not possible to turn off ClearType which means that everything viewed through the browser looks blurred.

This omission is a very significant issue for me in relation to websites and web-based products which are optimised for and/or can be accessed only via IE9 and or where IE9 is pre-installed (eg in the current versions of Windows Server 2008 for terminal server sessions).

Internet Explorer is, currently, the de facto browser in the business world. The implications, therefore, of not being able to browse/use web-related products because of the mandated use of ClearType in Internet Explorer do not need to be spelled out.

2. Embedded/subset text

There is an increasing practice for images of text, to which sub-pixel rendering (such as ClearType) has already been applied, being embedded/subset into web pages and/or documents.

Such text is therefore off limits to me because once it has been embedded/subset, it cannot be changed (other than zoomed) and is therefore blurred.

A good example of this would be tweets that are posted on, say, MSN.com.

Again, the implications of my not being able to view subset/embedded text on screen because it has been rendered in ClearType do not need to be spelled out. It is all the more frustrating because images of text which have been rendered without ClearType (or its equivalent) look fine, assuming that a suitable font is selected.

In comparison, text which is displayed on a TV screen (when watching television programmes/films etc) typically looks fine.

3. Font selection in web pages

I have noticed an increasing tendency for web pages to select fonts, such as Calabri and Segoe UI, which are designed for sub-pixel rendering.

However, the text looks blotchy to all users if viewed without font smoothing and, to me, it looks blurred, with standard font smoothing switched on.

I am aware that I can, for the moment, override the web page's font selection via browser settings (eg in Firefox). As such, this issue is not, for me, as serious as the issues identified in items 1 and 2 above.

My recommendations

  1. Browser/user agents should not mandate the use of sub-pixel rendering as is the case with IE9. The ability to turn off ClearType in IE9 (and future versions/releases of the browser) needs to be re-introduced. (If this has an effect on performance such as "hardware acceleration", so be it. At least, I will be able to look at the content when it is displayed). In the meantime, Microsoft should allow for operating systems such as Windows 7 and Server 2008, which come with IE9 pre-installed, to be rolled back to IE8.
  2. The practice of subsetting/embedding text to which sub-pixel rendering has already been applied should be discouraged/stopped because it makes the text in question off limits to me (and to people in the same situation as me).
  3. The increasing practice of web pages selecting fonts (such as Segoe UI) which are designed to be displayed via sub-pixel rendering should be discouraged/stopped. Web pages should select fonts which look fine with or without ClearType (or its equivalent) being used.


Sub-pixel rendering is causing problems.

I am therefore keen for W3C and the IT industry to acknowledge the problem and to work towards providing me and people in the same position with viable alternatives.

A post script - a linked issue

Whilst strictly outside of the scope of this paper, I have not been able to adapt to Windows 7/Office 2007.

From a user's perspective, I feel that this is because of the way in which Microsoft has integrated ClearType into its software, designed the graphical user interface on the assumption that, amongst other things, ClearType will be left on and tuned to the user's preference. Unfortunately, it is not simply a matter of turning off ClearType and personalising the settings. More radical forms of intervention have not worked either, such as changing the system fonts from Segoe UI to Tahoma (which involves going into the system registry with all that entails).

The best analogy which I can come up with is expecting an air-conditioning system, which is designed for open plan offices, to operate properly if the floor space is subsequently partitioned into offices. It can't.

I have therefore had no choice but to retreat to XP/Office 2003 with which I am comfortable. However, I am acutely aware that Microsoft is looking to pull the plug on XP/Office 2003 in the coming months, which is of great concern indeed as I have no alternative solution in place. I would, of course, be happy to submit a paper on this specific issue.

A second post script

Since writing this paper, I have had some constructive dialogue with Microsoft about the challenges which I am facing with IE9 and the Windows 7 set up. I am waiting for solutions.

I have also established that sub-pixel rendering is not deployed in devices which run mobile operating systems because this form of rendering does not work in relation to rotating screens. Consistent with this, I am completely comfortable with the display (out of the box) on devices such as the Samsung Galaxy SIII, the Nexus 7 tablet and the iPad. As far as I can discern, the lack of sub-pixel rendering does not put people off using them nor does it affect performance (eg speed of the browser on the device in question)