Does the teaching tool I want exist? Top-left pane shows editable HTML source. Bottom-left pane shows editable CSS. Right pane shows rendered view, and automatically updates whenever both the HTML and CSS panes are in legal (parseable) states, so that each valid edit is immediately displayed. (I know of many such tools that redisplay when told to, but I want polling + parsing + render-when-valid. I could build it myself, but I'm hoping it already exists.) thanks in advance