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Re: Images and alternative text

From: Philip Taylor <pjt47@cam.ac.uk>
Date: Fri, 08 Aug 2008 19:43:41 +0100
Message-ID: <489C93DD.1020002@cam.ac.uk>
To: Dave Singer <singer@apple.com>
CC: "public-html@w3.org" <public-html@w3.org>

Dave Singer wrote:
> There seems to be at least some assumption here that if I put Latex into 
> the alt text, UAs could reliably recognize it as such.  On what 
> warrant?  alt text is, well, text, isn't it?

I'm not assuming that UAs will recognise it - I'm just assuming users 
will recognise it and work out how to read it. It's not fundamentally 
different to assuming users will recognise and understand what "Σax²" 
means, except that it's a different choice of syntax.

(Some users won't understand one or the other or both of those syntaxes, 
just like some won't understand the surrounding English prose or the 
concepts being explained, and that's okay since I would only be writing 
equations for a specialised audience.)

> I think the real need to have accessible alt text, or if it's not 
> available, a damn good *expressed* reason for its lack (in suitable 
> brackets such as {}), really trumps a wish to put syntactically 
> formatted text in other un-identified languages into the alt tag.
> Yes, if you want to put Latex in there (or any other text), you'll need 
> to put something before an initial {;  perhaps something that helps you 
> diagnose that it is, in fact, latex source, and not really alt text at 
> all.  Perhaps, even
> alt="{latex} {x \over y} = {1 \over {y \over x}}"
> ?

According to the current HTML 5 spec (as I understand it), that means 
the image is a key part of the content, and there is no textual 
equivalent of the image available, and the kind of image is "latex} {x 
\over y} = {1 \over {y \over x}".

I believe the LaTeX code *is* a textual equivalent (because it's text 
(being a human-readable string of mostly-ASCII characters) and it has 
equivalent meaning to the rendered image without any complex encoding), 
so that meaning is inappropriate; and the extracted string between the 
outermost "{" and "}" is clearly not what was intended, and it's not the 
kind of image anyway. (The kind would be something like "equation", but 
that's thoroughly unhelpful to users who can't see the image.)

Something like alt="LaTeX: {x \over ... }" would avoid those problems, 
but still that's ugly (particularly if you have many equations in a 
single sentence) and redundant (since it's pretty clear when text is 
LaTeX, to readers who understand LaTeX) and it's not entirely obvious 
how to implement this correctly (as you demonstrated by using "{latex} 
{...}" which'll get badly misinterpreted :-) ).

Philip Taylor
Received on Friday, 8 August 2008 18:44:23 UTC

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