ReporterX wrote:Thanks for the reply.
Unfortunately what I want is "Ask per request" even in the same website that I would allow.
If I add the domain name to the "Accept from", it would be "Allow per site".
That is what I answered wait until NoScript 3
If you don't allow scripts on the third-party websites, and check all Embeddings (not necessarily "Apply these restrictions to whitelisted sites too") then I think NoScript will default display placeholders for youtube embeddings. It's when scripts are allowed on the 3rd-party site that you're stuck with either not having this or "Apply these restrictions to whitelisted sites too".
ReporterX wrote:About noscript.allowedMimeRegExp, let's say I want to apply these restrictions to yahoo.com only (but not other whitelist sites).
What should I type? It does not seem like this pref could do this job.

No, it can't act that way, because it's a whitelist of origin sites for plugin content, nothing to do with sites requesting the content.
Maybe ABE alongside that pref could do that, but again, I think the sort of feature you're looking for will likely be part of NoScript 3...