YouTube Link Title

Adds video thumbnail on mouse hover and embeds video on click.

Play videos in-line without opening new tabs and scroll page at the same time.

Also supported: Invidious, Vimeo, LiveLeak, Dailymotion, vidme, WorldStarHipHop, Vine, Coub, Streamable.

This script is mostly functional.

YouTube titles are disabled for now but can be enabled.

Feel free to fork (but change the YouTube API key).

If you left-click a video link, the video gets embedded and started.

Time offsets like "t=3m21s" are preserved.
You can still follow links normally by middle click, Ctrl + click etc.
Some uploaders don't want their videos to be embedded. These videos will open in a new tab.
Embed modes:

Support for Invidious (just invidious.osi.kr instance for now), Vimeo, LiveLeak, Dailymotion, vidme, WorldStarHipHop, Vine, Coub and Streamable
Supports rComments for reddit for every comment load.
Titles are cached to reduce traffic and delay on subsequent visits.
To change settings: Monkey toolbar icon → "User Script Commands" → "Set up YouTube Link Title..."

If link text doesn't contain a URL, the video title will be added as tooltip instead.
A crossed out link indicates that the video is unavailable.
Move cursor over links to see preview images.
URLs are rewritten to clean HTTPS addresses.

Just use a nice userscript like YouTube Peek Preview
If you really want it enabled in this script, then:
Uncomment lines 216, 217, and 219
replace the API key on line 657: after "String.fromCharCode.apply". You can create one in Developer Console using instructions from here
Before: net.json('https://www.googleapis.com/youtube/v3/videos?id=' + vid + '&part=' + part.join(',') + '&fields=items(' + fields.join(',') + ')&' + String.fromCharCode.apply(String, [107,101,121,61,65,73,122,97,83,121,68,87,120,79,114,52,76,105,52,65,54,72,116,89,120,57,107,55,98,86,98,67,81,54,56,118,112,119,74,74,117,111,99]), function(code, obj, txt) {
After: net.json('https://www.googleapis.com/youtube/v3/videos?id=' + vid + '&part=' + part.join(',') + '&fields=items(' + fields.join(',') + ')&' + 'key=NEWYOUTUBEAPIKEYHERE', function(code, obj, txt) { where NEWYOUTUBEAPIKEYHERE is your own API Key

Some embeds may fail to load with because of Content Security Policy:
Content Security Policy can be carefully overidden to allow very specific embeds on chosen websites
Use an extension like Content Security Policy Override very cautiously and at your own risk.