No, there is no block-level element for importance, and no element whatsoever for a summary (the summary
element inside details
is not really for a summary at all but rather a title of a kind).
In traditional HTML up to and including HTML 4.01 (and XHTML 1.0), em
and strong
indicate emphasis, but their definitions are very vague. In practice, they are little else than pseudo-semantic counterparts of italic and bolding, i
and b
. HTML5 makes the topic even more confusing by distinguishing between “stress emphasis” (which “changes the meaning of the sentence”) and “strong importance, seriousness, or urgency” without really defining the difference. In any case, em
and strong
are text-level (inline) markup, intended for individual words or short phrases rather than entire sentences, still less paragraphs or sequences of paragraphs.
The closest you can get is <p><strong>...</strong></p>
, but it is questionable whether this is a good idea. It’s surely not the way strong
is meant to be used, but it’s formally valid.
Thus, the practical approach is to use div
elements (or maybe p
elements) with a class
attribute and suitable styling.