Proper & complete way to declare website graphical-logo microdata

Question

I looked for the proper way to declare logo image in HTML5 with microdata

in this address: https://support.google.com/webmasters/answer/146861?hl=en also by clicking Microdata (recommended) sublink it can be seen that

<img alt="logo" src="http://www.example.com/logo.png" />

became

<img itemprop="logo" src="http://www.example.com/logo.png" />

So my 1st question is why they omitted alt tag. What if I code as below. Technically it'll be still correct, right? Or is there any conflict between alt tag and itemprop="logo"?

<img itemprop="logo" alt:"Image with a sea blue background and tale character Little Black Fish jumps out of the water" src="http://www.example.com/logo.png" />

my 2nd question is I have a complex, great-in-size logo and with my logo image I want to tell to search engines the story of my logo. (the symbols, their meanings, creators of that symbols etc.)

So which micro data tag I need with which syntax to achieve my aim if any? thanks

Answer 1

That’s a very bad example from Google:

1. The alt content "logo" is usually not appropriate for a logo (unless the company/etc. is named "logo").
2. An img with Microdata’s itemprop attribute still needs an alt attribute.

There are no conflicts between alt and itemprop. Your example with both of these attributes is fine.

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To describe your logo in greater detail, you would need to find an appropriate Microdata vocabulary. If you want to use the schema.org vocabulary, the type ImageObject is probably the best you can get currently.

You could also use the longdesc attribute (currently a Working Draft) "to link descriptions to images in HTML5 content".