This is skin specific, as apart from ‘empty.htt’, all templates are under the control of the skin developer. Though you can of course modify them where allowed.
For page templates, like About.htt, such a feature can be implemented as already described by David.
JSmartTextArea textAreaName = new JSmartTextArea(4, 20).setAllowHTMLEditing(true).setFullHTMLEditing(true).spelling();