Strictly speaking, you should put something that makes sense - according to the spec here, the most correct version is:
<input name=name id=id type=checkbox checked=checked>For HTML, you can also use the empty attribute syntax, checked="", or even simply checked (for stricter XHTML, this is not supported).
Effectively, however, most browsers will support just about any value between the quotes. All of the following will be checked:
<input name=name id=id type=checkbox checked><input name=name id=id type=checkbox checked=""><input name=name id=id type=checkbox checked="yes"><input name=name id=id type=checkbox checked="blue"><input name=name id=id type=checkbox checked="false">And only the following will be unchecked:
<input name=name id=id type=checkbox>See also this similar question on disabled="disabled".
