Can the alt attribute be omitted without hurting accessibility?
In the current editor's draft of HTML 5, it is suggested that the alt attribute for img elements should no longer be required. The reasoning is that in some cases alt text will be omitted, regardless of whether it is required or not, so it might as well be made optional. Otherwise some authoring tools will automatically insert empty, repeated or meaningless alt text. At least that's how I understand the reasoning explained in Why the Alt Attribute May Be Omitted.
Basically it seems to come down to "Many people and many tools aren't using the alt attribute properly, and adding alt text to images uploaded to online photo galleries like Flickr is too much work for end users and tool developers, so the alt attribute should be made optional." Is that really the best solution though?
To help authors write useful alt text, the HTML 5 specification should provide guidance on that. I am pleased to see that the section describing the img element in the current draft does that well, except for the first example which seems a bit too long (and contains information that all users would benefit from). The rest of the examples are good. They could be worked on, but they are good.
Despite that guidance, there will always be cases where alt text is missing, either because of broken authoring tools or because of author ignorance or laziness. So the specification needs to define how user agents should handle missing alt attributes. In no way does that necessarily mean that images with missing alt attributes should be valid HTML 5.