Consequences of not using a lang attribute in an html5 html tag

Question

If at the start of my html5 doc I simply use:

how will my site be at a disadvantage for not using something li

Answer 1

Good question, but I think most of what you want to know is simple reference:

http://www.w3.org/TR/html4/struct/dirlang.html

Some highlights:

Language information specified via the lang attribute may be used by a user agent to control rendering in a variety of ways. Some situations where author-supplied language information may be helpful include:

• Assisting search engines
• Assisting speech synthesizers
• Helping a user agent select glyph variants for high quality typography
• Helping a user agent choose a set of quotation marks
• Helping a user agent make decisions about hyphenation, ligatures, and spacing
• Assisting spell checkers and grammar checkers

I don't believe there are any important differences between HTML4 and HTML5 regarding this.