Grab CardSpotter from the Chrome Web Store. here
Just to get a rough idea of how it works - watch the short demo. here
After the video has started you can enable CardSpotter. You do that by clicking the toolbar icon.
Tooltips should now automatically appear as cards are played.
If they do not - click the card you want a tooltip for.
When CardSpotter is enabled a small magnifying glass and a dropdown menu is shown in the lower right of the video.
If you do not start the video before you enable CardSpotter there is no video information available and CardSpotter will most likely complain that the video resolution is not optimal.
If you want to temporarily disable CardSpotter to perhaps change some video settings - click the magnifying glass with the - sign to minimize it. Click it again to bring the menu and CardSpotter back.
CardSpotter works by capturing mouse clicks using an invisible overlay. The relative size of this can be configured under the extension options.
The extension options can be accessed by right-clicking the CardSpotter toolbar icon and selecting "Options".
Either you check out some more examples that are known to work relatively well in the examples section of the DOCUMENTATION (both twitch/youtube).
Or play around with it on one of your favorite streams. But please, do let me know how it goes. Either by rating the extension if everything is awesome, or if something is broken by emailing me at firstname.lastname@example.org so that I have a chance to fix it.