Essentially you need to use the selection api to move the insertion point over each word to get Safari to highlight it. Here's an example to scan over the first thousand words...
textarea = document.getElementById("mytextarea");
textarea.focus();
var selection = window.getSelection();
selection.modify("move", "backward", "line");
for (var i = 0; i < 1000; i++ ) {
selection.modify("move", "forward", "word");
}
// Remove focus from the element, since the word under
// the cursor won't have a misspelling marker.
textarea.blur();
This code was lifted from the WebKit Layout test suite.
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