In the strictest sense, yes, you always have to have contrast of 4.5:1 or more. But there are exceptions and a nice can of worms waiting.
If your drop shadow or text outline have a contrast of 4.5:1 to text fill, that might be enough. Though this depends on the location of your drop shadow and width of your text outline. Drop shadow at the bottom of the text or miniscule outline doesn't increase readability that much.
You can get away with smaller contrast if your text is large enough, then the contrast of 3:1 is sufficient.
There are two success criteria to consider in addition to 1.4.3 Contrast (Minimum) / AA. The other success criteria are 2.2.2 Pause, Stop, Hide / A and 2.3.1 Three Flashes or Below Threshold / A.
2.2.2 Pause, Stop, Hide states that:
For any moving, blinking or scrolling information that (1) starts automatically, (2) lasts more than five seconds, and (3) is presented in parallel with other content, there is a mechanism for the user to pause, stop, or hide it unless the movement, blinking, or scrolling is part of an activity where it is essential[.]
Being a background video, it isn't in any way essential to the activity of the page.
If you are going to get sufficient contrast between the text and the background only on some frames, you now have both moving background and a blinking text on top that background for some users.
This leads to 2.3.1 Three Flashes or Below Threshold, which states:
Web pages do not contain anything that flashes more than three times in any one second period, or the flash is below the general flash and red flash thresholds.
Success criteria notes also that:
The terms "blinking" and "flashing" can sometimes refer to the same content.
- "Blinking" refers to content that causes a distraction problem. Blinking can be allowed for a short time as long as it stops (or can be stopped)
- Blinking usually does not occur at speeds of 3 per second or more, but it can. If blinking occurs faster than 3 per second, it would also be considered a flash.
So, if you want your site to be accessible but can't overturn autoplay decision, there are some solutions.
- Add contrast by using text outlines or colored boxes behind the text. (I think gradients are ok but make sure that contrast is sufficient in every point.)
- Add a way to stop the background video. Do make sure that it is accessible.