I disagree. People know that they can't change the contents of a greyed out box. Less savvy people may see a box with black text and think that there is something wrong with their computer, keyboard or the web page if they can't click in/edit the contents.

I would say that, at first, most people do NOT know that greyed-out boxes can't be changed. They only know after attempting to click on it a few times.

The key is to make it not look like an edit box, then people won't think they can interact with it. You can see it in practice here at HN: edit a submission and you can see that the URL is not in a text-box, so the text can appear like any other text on the page.

In general, making things harder to read can't be good, and should never become a standard.

> In general, making things harder to read can't be good, and should never become a standard.

It reminds me of comments here on HN which have very low scores: they get greyed out and effectively "disabled", like a disabled form field.

However, disabled form fields which demand no interaction, not even reading, from the user should not attract interest. It's fine to make fields which are unimportant less prominent.

As you say, real read-only content (such as the text on a web page) should both be easy to read, and not shown inside a user input control in the first place.

"disabled form fields which demand no interaction, not even reading, from the user should not attract interest"

This would be an argument to not show them at all. Requires no interaction, conveys no information and doesn't attract interest? Then it is only cluttering your UI, consider removing it.

Which is why the third option, removing any appearance of a control, can be nice. If I can't do anything with it, why style it as anything other than static content?

Also consider that if things need to be read, high contrast is better. I think github, as an example, suffers from low contrast colors on many things, the file list being one; it's relatively hard (I need to put more effort into it than I think should be necessary) to see on every LCD screen I've tried it on. Obviously, I can fix this myself using greasemonkey or user stylesheets or something, but it seems to be a growing trend.

Making unusable inputs harder to read is an affordance that makes it easier to scan the page for things that are usable. If your page has a scattershot of usable and unusable controls, the problem is probably more fundamental than how you grey things out.