The spellcheck attribute tells a browser whether or not to check the spelling/grammar of the text
in an editable element. This won't affect validation.
Typical handling is underlining problem words/phrases with red and offering contextual menu help.
In Chrome 10+ (tested 13 and 17), a red underline only shows up when and incorrectly spelled word is typed for the first time and only when input is in focus.
Just "spellcheck" isn't enough as an attribute (it's invalid). You have to set true or false.
Inconsistent results in Safari 4 and 5 - sometimes it would check-as-typed with spellcheck=false and sometimes not with exact same form HTML.
Opera 10.53 doesn't spellcheck inputs regardless of setting and does check textareas regardless of setting.
Opera 11 reports it doesn't accept the attribute but honors it on both inputs and textareas.
History, discussion, and older browser support here.
IE 9 accepted the attribute but didn't do anything with it.
Android doesn't have spellcheck so it's Not Applicable.
