Text-shadow in WebVTT not working

[Peterdoo]

Have you read the FAQ and checked for duplicate open issues?
YES

What version of Shaka Player are you using?
v4.3.4-uncompiled

Can you reproduce the issue with our latest release version?
YES

Can you reproduce the issue with the latest code from main?

Are you using the demo app or your own custom app?
Demo app shaka-player-demo.appspot.com

If custom app, can you reproduce the issue using our demo app?

What browser and OS are you using?
Chrome Version 109.0.5414.120 (Official Build) (64-bit) on Win11

For embedded devices (smart TVs, etc.), what model and firmware version are you using?

What are the manifest and license server URIs?

What configuration are you using? What is the output of player.getConfiguration()?

Demo app shaka-player-demo.appspot.com

What did you do?

Display the following WebVTT cue:

WEBVTT

STYLE
::cue(.red) { color: #FF0000FF; }
::cue(.yellow) { color: #FFFF00FF; }
::cue(.cyan) { color: #00FFFFFF; }
::cue(.white) { color: #FFFFFFFF; }
::cue(.bg_black) { background-color: #40; text-shadow: -2px 0 10px #000000FF, 0 2px 10px #000000FF, 2px 0 10px #000000FF, 0 -2px 10px #000000FF; }
::cue { background-color: #00000000; opacity: 1; }

01:01:20.000 --> 01:01:25.575 line:90%,end align:center position:50%,center
<c.white.bg_black>This subtitle:</c>
<c.white.bg_black>WHITE </c><c.red.bg_black>RED</c>
<c.yellow.bg_black>YELLOW </c><c.cyan.bg_black>CYAN</c>

What did you expect to happen?
Text characters should be surrounded by the black non-transparent shadow. Shadow color: #000000FF (black with alpha=FF). The following picture has been made by putting the above STYLE into a css and displaying the cue as html:

What actually happened?

There is no shadow around the text characters.