4. Define logo alt text: to improve the usability and compatibility of the page

It worth mention that this also relates to SEO.

5. Define logo size: enhance efficiency of the browser.

Some people argued that it is not convenient for content maintenance. Perhaps it can be fixed by defining size in CSS.

6. Close all img tags for logo

We should not forget the standard of building a website, for those who are not familiar with W3C, here are two specs.

HTML 4.01 Specification
CSS 2.1 Specification

7. Compress logo with Gzip? Forget about it!

For logos that would stay for long time, we can set a longer cache to reduce requests to server. It is useless to compress with Gzip on server side, but different case for css/js/html documents, and it is compulsory if server pressure allows. (most css/js/xml document sizes can be compressed up to 70-80%)