website design - Are italics on the web bad for accessibility?

On a current web project, we've received word from the client that we can no longer have any italicized text on the website we are building.

When I asked for some reasoning, I was told that "our accessibility person says it's bad for accessibility", with no other justification.

Honestly, I've never heard of this before, so I tried to do a bit of research on it.

All I've been able to find is this post from 2007 which literally comes to the conclusion of:

I never found the text of any study that proves it. There's reference to one, and that's good enough for me. Instead, I am relying on the "everyone says it" method

With most of the post's references having long since rotted away.

As far as I can tell, the main reason italics were ever an accessibility issue were low dpi screens making small oblique fonts unreadable.

But in this retina world, I find it hard to believe that's still a valid reason.

So, what are the reasons for italics being bad for accessibility, and are those reasons backed by any real studies?

Answer

Italics are a known problem for some people with dyslexia and the general advice has been to avoid italics (particularly large blocks of italic text) and instead use bold for emphasis.

The British Dyslexia Association says:

Avoid underlining and italics: these tend to make the text appear to run together. Use bold instead.

UX Movement touches on this in an article about Bad Practices That Hurt Dyslexic Users:

Italics are sometimes used to highlight text. But you shouldn’t use italicized text because they make letters hard to read. The letters have a jagged line compared to non-italic fonts. The letters also lean over making it hard for dyslexic users to make out the words. When the text size is small, the text is even more illegible. A better way to highlight is to use bold text because the letters are clearer and give better contrast.

There was an experimental study in 2013 looking at which fonts were easiest for dyslexic individuals to read:

Based on the evaluation of 48 dyslexic subjects ages 11-50, reading 12 texts with 12 different fonts, they determined that reading performance was best with sans serif, monospaced, and roman fonts used in the study. They also found that reading was significantly impaired when italic fonts were used.

In WCAG, for Guideline 3.1 Readable (Make text content readable and understandable) there is an advisory technique for "Avoiding chunks of italic text".