The global spellcheck attribute tells a browser whether or not to check the spelling/grammar of the text
in an editable element.
Values include "true" and "false", and are required if thespellcheck attribute is present.
When the spellcheck attribute is not included, the default value depends on the element and browser. Generally, contenteditable and <textarea>s behave as if it were set to "true", and <input>s behave as if it were set to "false".
Spellcheck does not impact validation.
Typical browser handling is underlining problem words/phrases with red and offering contextual menu help.
Spell checking is NOT done on page load or on blur in most browsers.
In IE10, Chrome and Opera, the spelling of words is only checked when the the word is completed (by spacing or adding punctuation after the word). In most browsers spellchecking occurs when first typed for the first time, but only when the user adds a space or punctuation after a misspelled word. Tabbing out of the contenteditable area will not cause the word to be spell checked.
Safari is similar to Chrome and Opera, but also enhances spell checking by providing autosuggestion when the attribute is included and set to "true".
In Firefox, spell check also happens on blur. And, when spell checking is conducted on that one element, Firefox spell checks the rest of the document, including areas that have spellcheck set to "true" and elements that are by default spell checked.
History, discussion, and older browser support here.
Android provides autosuggest, but doesn't support spellcheck.