I think it would be unexpected that 'content: none' paints more
(backgrounds, borders) than 'content: normal'. I suggest changing
# A margin box is instantiated if and only if the computed value of
# its ‘content’ property does not match that of its initial value.
| A margin box is instantiated if and only if the computed value of
| its ‘content’ property is not ‘normal’ or ‘none’.
Also, this section should clarify what "instantiated" means.
Add a paragraph
| A margin box that is not instantiated does not paint anything.
Finally, should the margins, borders, padding and width/height box
not-instantiated box still take up space for the purpose of Variable
Dimension Computation Rules? I don’t feel strongly either way. If not,
extend the paragraph
| The used values of its 'margin-top', 'margin-bottom', 'margin-left',
| 'margin-right', 'border-top-width', 'border-bottom-width',
| 'border-left-width', 'border-right-width', 'padding-top',
| 'padding-bottom', 'padding-left', 'padding-right', 'width' and
| 'height' are all set to zero.
(I’d like to abbreviate this. Is 'margin-*' suitable for spec text?
Shorthands normally don’t have used values. Maybe "box model
properties", with a reference to CSS21:8?)