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It seems, with http://drupal.org/node/323844 fixed, too many CSS files from parent theme are incorrectly included in the editor area.
In my case, when viewing a node, these CSS files are included:
The editor area incorrectly added these: (by inspecting in Firebug)
ballzen.info stylesheet override looks like:
; You can also override any of Zen's stylesheets or any module's stylesheets,
; an /extremely/ useful feature. See the excellent Drupal 6 Theme Guide at
; http://drupal.org/node/171209 for more details.
stylesheets[all] = layout.css
stylesheets[all] = html-elements.css
stylesheets[all] = ballzen.css
stylesheets[print] = print.css
; To prevent stylesheets of a base theme or of a module from being included in
; our sub-theme, we specify it in our .info file (and we don't need to have a
; stylesheet in our sub-theme with that name.) For example, we prevent the
; zen.css file in the base theme from being included by specifying it here.
stylesheets[all] = zen.css
stylesheets[all] = image_attach.css