# Re: Images and alternative text

From: Philip Taylor <pjt47@cam.ac.uk>
Date: Fri, 08 Aug 2008 14:43:49 +0100
Message-ID: <489C4D95.2070902@cam.ac.uk>
To: Ian Hickson <ian@hixie.ch>
CC: "public-html@w3.org" <public-html@w3.org>

Ian Hickson wrote:
> On Thu, 7 Aug 2008, Philip Taylor wrote:
>>   <img src="..." alt="{x \over y} = {1 \over {y \over x}}">
>
> ...would be a horrific alternative text to give a screen reader.

I'm not aware of any better alternatives for the general case. Text like:

>    <img src="..." alt="The fraction x over y is equal to 1 divided by the
>    fraction y over x.">

is clearly unscalable when you have non-trivial equations.

Since these LaTeX-to-HTML tools are primarily aimed at sighted users
with graphical browsers, and only a tiny fraction of users will rely on
screen readers, it doesn't make sense to spend significant effort
optimising the output for screen readers. The tools can't be expected to
ask authors for a textual alternative to each equation (because the
whole reason for this new alt stuff is that tools often can't do that
and have to make do with the limited information and intelligence they
have). So the tools will carry on doing the best they can do that
doesn't take much effort, which is to put the LaTeX source of the
equation in the alt attribute. Given that that situation seems unlikely
to change, it's a problem if HTML 5 adds features that will trip people
up by reserving certain syntax and making it non-conforming to match
that syntax when supplying real alternative text.

(If it wasn't non-conforming, then there would be less of a problem -
people could write <img alt="{x \over y}"> with no worries other than
that it might be rendered slightly differently than other equations by a
few UAs, which isn't a major issue; or they could write <img alt="{LaTeX
equation: {x \over y}}"> so it'd at least be handled consistently for
all equations. But HTML 5 doesn't allow either of those, so the
conformance requirements are a problem for people who care about
conformance. But making it conforming still wouldn't solve the problem
of some UAs misidentifying the equations as critical images with no
attempt at proper alternative text.)

> I don't think it's equivalent to the image at all. It's the source of a
> program that was used to generate the image, but that's not the same
> thing. Would you consider the replacement text of a fractal to be the C
> source code that generated it? Or the replacement text for an SVG file to
> be the raw source code of that SVG file?

The mapping between source and output in those cases is a very
substantial change - the text "<circle>" and the graphical rendering of
a circle are very different, and information that is obvious in one
format can be almost entirely hidden in the other. The mapping between
LaTeX source and output is far more straightforward - the text "{x^2
\over y}" and the graphical rendering

2
x
---
y

are quite similar, and it doesn't get much worse when the equations
become more complex. The changes are just in layout, and in removal of
some symbols, and in rendering of some symbols as special shapes -
there's no revelation of hidden information in one form or the other[*].
So I don't think the analogy works.

[*] (except in some quite rare cases of mathematical jokes, like
"\lim_{\omega \rightarrow \infty} 3 = 8", which only make sense in the
graphical form)

--
Philip Taylor
pjt47@cam.ac.uk

Received on Friday, 8 August 2008 13:44:33 GMT

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