By David Sloan, published on 28th July 2006.
The concept of Web page linearisation is an important aspect of Web accessibility, and one well worth understanding, as it helps you understand how a Web page would be read out to someone who can't see it.
"Linearisation" refers to the way a Web page is transformed from a visual object with columns and rows of information into what is effectively one long line of text - which is what would be read aloud by a screen reader or shown by a text-only browser.
If you're sighted, and using a browser like Internet Explorer, Netscape or Firefox, you'll see the whole Web page, unless scrolling is required to reveal content towards the bottom of the page (or occasionally towards the right hand side). You can very quickly scan across the page to find the main content - the important information on the page, ignoring any decorative content or repetitive content (like the navigation bars present on each page of the University web site).
A typical layout of a Web page is shown in Figure 1.
Figure 1 - Typical Web page layout.
However, for someone who can't see, and who relies on software to read out the page, the experience is quite different. With audio, the page content has to be read out in a linear fashion (i.e. line by line). You can't quickly scan backwards and forwards across the page - you must wait to hear what is present, and only once you've heard a line of text are you aware that it's there. Think of it as someone speaking the content of a Web page - all of it, in order! - to you over the phone.
The same is true for browsers that don't show columns or tables of text - again, the page content is linearised and shown as lines of text.
As you can probably imagine, receiving a Web page in linearised form can significantly extend browsing time. In the example of our typical Web page, the reading order of the page is shown in Figure 2.
Figure 2 - Order in which a screen reader 'linearises' page content.
Figure 2 shows that a screen reader user wanting to hear the main page content must first listen to the logo, header, main navigation and sub navigation. If each of these areas has many links, this listening process will take some time, possibly several minutes. You can imagine that the cumulative effect of this over several pages will be one of extreme frustration, especially as much of the content the user is forced to listen to will be the same from page to page.
This means that one major goal of accessible Web design is about trying to make browsing the page in linearised form as efficient as possible - which will at the same time help improve the page's keyboard accessibility.
We've provided some suggestions in the article Improving Keyboard Accessibility.
If you have the appropriate software, you can quickly see how your pages linearise by:
Figures 1 and 2 in this page have been provided with text descriptions on separate pages, using the HTML longdesc attribute. However, as browser support for this feature is not very good at the time of writing, here are links to the two descriptions: Description of Figure 1 and Description of Figure 2.
Times Higher Education, Student Experience Survey 2012