There is a lot of bad HTML information floating around on the 'net in forums, on websites, and elsewhere, often put forward as "expert" advice. I once saw this rather erroneous information on a forum:
"Alt tags are those HTML codes you will see as <ALT=blahblahblah> -- what the ALT tag does is help people who are visually impaired or blind to 'see' webpages."
The first thing to note about ALT tags is they are not "tags" at all. The alt (short for alternate text) attribute is just that, a tag "attribute". Attributes often appear within HTML tags after the tag name; something like this:
<tagname attribute attribute="value" attribute="value1, value2">
The alt attribute is used in the img (image) tag to provide alternate text that shows when the image fails to load. The value (text) contained in the alt attribute is also used by web page reading software (for the visually impaired) and by many search engines when indexing page content. An image tag with an alt attribute might look something like this:
<img src="http://somedomain.com/afolder/someimage.jpg" width="250" height="180" border="0" alt="This is the alt text">
Using properly constructed alt attributes in your image tags can help with search engine optimization (SEO) by providing relevant keyword-rich text that most search engines will index, and it may benefit your website in additional ways. If the picture fails to load the text contained in the alt attribute will be displayed. This should help inform the viewer what they might otherwise be seeing. And, of course, it helps the visually impaired when using web page reading software.
The forum post referenced above went on to say:
"USE CAPITAL LETTERS IN THE ALT TAG SO THE SPIDERS READ IT AS A TITLE. IT GIVES IT MORE WEIGHT!"
The person posting this meant well. They were obviously attempting to help someone. But this advice is simply not true. And, in most cases, bad advice is worse than no advice. It isn't capital letters, or even bold text, that makes text a title (actually a "heading") and which may give it more weight, or importance, with the spiders. But that's for another article.