#-starting IDs are not found in the page

[dialtone]

$('<li id="#foo">hi</li>').appendTo("body"); $("##foo")

Doesn't find the just added li item.

[aercolino]

# is not a valid id starter: When defining fragment identifiers to be backward-compatible, only strings matching the pattern [A-Za-z][A-Za-z0-9:_.-]* should be used

see:  http://www.w3.org/TR/html4/types.html#h-6.2

[john]

aercolino is correct. We only accept IDs that are valid both in HTML documents and for CSS selectors.

[aheimlich]

Replying to john:

Just to clarify, this means that you should should not use #, ., or : in your IDs as they are a part of CSS selector syntax, so the pattern becomes this:

[A-Za-z][A-Za-z0-9_-]*