xhtml validation error - alt attribute missing from html image tag of svg format

[mixmastamyk]

With html output of images or figures (only in .svg format), the attribute alt of the img tag is missing, which is required under xhtml; causing validation errors. The same did not happen under the parent docutils html writer, from which it is derived from. The problem class is HTMLTranslator:

• Sphinx: https://github.com/sphinx-doc/sphinx/blob/master/sphinx/writers/html.py#L468
• Docutils: https://sourceforge.net/p/docutils/code/HEAD/tree/trunk/docutils/docutils/writers/html4css1/__init__.py#l1044

Docutils, (and Sphinx with other img formats) adds the alt attribute with a default value of the source path, which looks like a decent fallback. I've copied that behavior in my fix.

[mixmastamyk]

In short it goes from this:

<img src="../_images/spiral_model.svg" />

to this:

<img alt="../_images/spiral_model.svg" src="../_images/spiral_model.svg" />

In order to validate properly, and be consistent with other formats, such as .png images.

[mixmastamyk]

Looks good.