Success Criterion 1.4.10 Reflow (Level AA): Content can be presented without loss of information or functionality, and without requiring scrolling in two dimensions for:
- Vertical scrolling content at a width equivalent to 320 CSS pixels;
- Horizontal scrolling content at a height equivalent to 256 CSS pixels.
Except for parts of the content which require two-dimensional layout for usage or meaning.
As you may be aware of, certain group of low vision users use browser zoom functionality to see the web content. However, this sometimes might be difficult to view the content by scrolling in both horizontal and vertical directions. To put it simpler, it takes lot of time and effort to read the web content by scrolling into both the directions. This is kind of problem.
WCAG 2.1 introduced the new checkpoint called “1.4.10: reflow” to address such problems for the low vision users. This success criterion requires Content to be presented without loss of information or functionality, and without requiring scrolling in two dimensions when the content is zoomed up to 400%. When authors follow this success criterion then entire web content reflows in one single column without horizontal scrolling at 400% zoom. This enables low vision users to read the web content smoothly without horizontal scrolling.
Another important point to remember is that reflow should happen only at 400% zoom of screen resolution 1280px by 1024px but reflow need not to happen at 400% zoom of any screen resolution. This means that 400% zoom at screen resolution of 1280px(width) by 1024px(height) is nothing but 320px(width) by 256px(height). To put the things simpler, authors need to make sure that content should reflow either at 320px by 256px screen resolution or at 400
% zoom of 1280px by 1024px screen resolution. Basically, if your site is responsive then this checkpoint is almost going to be satisfied
This checkpoint is not applicable for the content that requires 2-dimensional scrolling. The reason is that such content will not deserve the meaning if it reflows in one direction. Examples of such content are data tables, videos, graphs, and so on.
Complimentary info on 1.4.10: reflow
The success criterion applies to both horizontally and vertically written languages. When 400% zoom is done on the horizontally written language content that scrolls vertically(ex: English)then it should not have horizontal scrolling. Similarly, when 400% zoom is done on the vertically written language content that scrolls horizontally then it should not have vertical scrolling. For the content that is horizontally written languages, the content should reflow vertically at 320px(width). For the content that is vertically written languages, the content should reflow horizontally at 256px(height).
It is often misunderstood that 1.4.4-resize text and 1.4.10-reflow are kind of similar, but both are different concepts. 1.4.4-resize text talks about all the content and functionality should be available at 200% zoom whereas 1.4.10-reflow talks about the all the content and functionality should be available at 400% without horizontal scrolling.