How does video embedding work? What URLs are recognized? Is it possible to link to a particular time in a video? Is video embedding also enabled on this Meta site?
(Posting this as a sort of sandbox question, since there doesn't seem to be one on any other SE that has embedding enabled. See this Meta.SE question for more context.)
How does video embedding work?
A link to a YouTube video, where the link text is the same as the href (typically a bare URL, but it can also be done using Markdown syntax) will be converted to an embedded video.
Is video embedding also enabled on this Meta site?
Yes. See the end of this answer for an example.
What URLs are recognized?
The following URL formats appear to be recognized:
Youtube's main domain:
http://youtube.com/watch?v=rEWaqUVac3M
https://youtube.com/watch?v=rEWaqUVac3M
Youtube's www subdomain:
http://www.youtube.com/watch?v=rEWaqUVac3M
https://www.youtube.com/watch?v=rEWaqUVac3M
Youtube Mobile "m" subdomain:
http://m.youtube.com/watch?v=rEWaqUVac3M
https://m.youtube.com/watch?v=rEWaqUVac3M
Youtube's Url Shortener:
http://youtu.be/rEWaqUVac3M
https://youtu.be/rEWaqUVac3M
There is no other standard or common url that Youtube uses at this time.
Is it possible to link to a particular time in a video?
Yes. You can directly use the URL that YouTube provides when using the "Copy video URL at current time" functionality from the video's right-click menu, which is the short video URL with ?t=XX appended (where XX is the offset in seconds). You can also use the ?start=XX parameter, or the ?t=XmYYs (minutes and seconds) form. When adding a time offset to a video that already has a query string (the section starting from the question mark), use an ampersand & instead of a question mark.
For example, the following URLs are all equivalent:
https://youtu.be/rEWaqUVac3M?t=110
https://www.youtube.com/watch?v=rEWaqUVac3M&start=110
https://www.youtube.com/watch?v=rEWaqUVac3M&t=1m50s
and will all result in the following embedded video cued to the appropriate time:
In direct links, YouTube normally supports specifying the start time in a fragment identifier as well (#t=XmYYs), but SE doesn't recognize that form for video embeds.