Source Order Matters

Strawberry, balsamic, black pepper sorbet
A picture of my strawberry, balsamic, black pepper sorbet, which makes sense later in the post and because my blueberry sorbet didn’t come out so well.

CSS is providing newer and more complex methods of laying out your pages. Given the multiple form factors a responsive site has to support, it makes sense that developers want easy ways to structure the layouts that aren’t all floats, clears and position: absolutes.

Regardless of how you want your layout to appear in a browser, you must keep in mind that a clear HTML structure is important to search engines. Sorry, while the bit about search engines is true, it’s not really what I consider important, but it is more likely to get some people to pay attention.

What I consider important are users of screen readers or any kind of assistive technology that allows a user to navigate a page by regions (typically by the Tab↹ key), as well as sighted keyboard users.

WCAG Conformance

WCAG isn’t the final word in making things accessible, but it is the baseline. As such, technique C27, making the DOM order match the visual order, is a good to one to know. It applies specifically to Success Criterion 1.3.2 (Meaningful Sequence) and Success Criterion 2.4.3 (Focus Order). Each of these is Level A, which means they are necessary to satisfy for even the lowest level compliance.

The technique offers some context for how failure to maintain consistent source order can be confusing to users:

This could be due to the user switching off the author-specified presentation, by accessing the content directly from the source code (such as with a screen reader), or by interacting with the content with a keyboard. If a blind user, who reads the page with a screen reader that follows the source order, is working with a sighted user who reads the page in visual order, they may be confused when they encounter information in different orders. A user with low vision who uses a screen magnifier in combination with a screen reader may be confused when the reading order appears to skip around on the screen. A keyboard user may have trouble predicting where focus will go next when the source order does not match the visual order.

That might be a little too much to quickly parse, so let me try…

An Attempt at an Example

Given the average developer’s love of using lists (ordered or otherwise) for everything on a page (assuming he or she has outgrown divs), let’s consider what happens when you use a list in your markup and then style it using a grid.

In the case of an ordered list, like a set of instructions, the order is very important and the browser will convey that to assistive technology. Borrowing a bit from Greg Whitworth (stealing, really), imagine a (very minimal) recipe:

  1. Puree the blueberries
  2. Push through a sieve to extract the skins
  3. Put in ice cream maker for 20 minutes

Visually as a two-by-two grid the CSS could cause that to flow top to bottom or left to right. Regardless of how it looks on screen, the order of the list must be retained. If a developer who is struggling with the CSS tries to just fudge the HTML (because the source order doesn’t matter for CSS grid or flexbox), you might get this list:

  1. Puree the blueberries
  2. Put in ice cream maker for 20 minutes
  3. Push through a sieve to extract the skins

I can tell you that this sorbet will not come out well.

The Takeaway

Code your pages so the HTML source linearizes well and the content makes sense without styles. When you do add visual styles, try to get them to conform to the source order as much as possible.

CSS grid and flexbox can make it tempting to render something completely differently than some bad structure you might inherit, but take a moment to think through its impact first.

What Did It This Time?

Mozilla Hacks posted a great article on CSS grids that had one statement that caused the hairs on the back of my neck to stand up and panic:

One aspect of it is that the ordering of the content in the HTML markup isn’t important anymore.

My fear was that developers would read that and take it to mean that the structure of their HTML was now no longer a concern. I wasn’t the only one who had that reaction. It came up on the CSS Working Group conference call this morning when Brian Kardell asked the question:

bkardell_: Effectively I guess…do we need to strongly pusht he same sort of understanding to authors that we’re pushing to the tooling? Even just today Moz hacks posted an article saying that the order of content in HTML markup isn’t important anymore

It also came up on the Gitter w3c/a11ySlackers channel. This is good because it means people are paying attention.

By the time I started this post, Mozilla Hacks approved comments from Dylan Barrell, fantasai, and Greg Whitworth and subsequently updated the post with this:

One aspect of it is that the visual order of elements on the page is decoupled from the order of elements in the markup. That’s important because on a page the source order is used for things like speech and tab navigation, so CSS Grids allow you to optimize the markup for accessibility without compromising your ability to manipulate the visual result.

Given that the original post isn’t about accessibility, I think it’s fair to leave it at that. But since you are here, bear in mind C27 above.

Update: October 25, 2015

I’ve just written a post, HTML Source Order vs CSS Display Order, which shows how different CSS layout methods can affect visual display. You can consider them examples for this article. Below are some images showing the effects of Flexbox and Grid.

CSS Flexbox tab order in Chrome 45.
Animation of Chrome 45 tabbing through flex items.
CSS Flexbox tab order in Firefox 41.
Animation of Firefox 41 tabbing through flex items.
CSS Grid tab order in Internet Explorer 11.
Animation of Internet Explorer 11 tabbing through CSS Grid items.
CSS Grid tab order in Firefox 41 with the flag enabled.
Animation of Firefox 41 tabbing through CSS Grid items. The flag has been enabled, but as you can see support is pretty poor.



Thanks for posting this. I wrote up my thoughts recently about Grid Layout, specifically in terms of the issue of flattening out of mark-up in order to make everything a child of the grid. It’s for this reason I believe that subgrid is an important part of the spec.


[…] The future of layout with CSS: Grid Layouts but remember that Adrian Roselli told you that Source Order Matters. […]


[…] Source Order Matters – Adrian Roselli – […]


[…] […]


[…] month in my post Source Order Matters I wrote about why we need to consider how the source order of the HTML of a page can affect users […]

Leave a Comment or Response

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <s> <strike> <strong>