If TTML is implemented natively in a user agent, it could expose fingerprinting vectors that aren't otherwise exposed. The spec should mention this risk so native implementations know to make intentional choices. The things I noticed to call out are:
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Anything that's "implementation dependent" might be a fingerprinting vector.
- Many initial style values are defined by the specification, so they wouldn't reveal anything. However,
<tts:color> and some others are described as implementation-dependent.
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A user's preference for how fast they consume media (e.g. 2x vs 2.5x).
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The request for timed text indicates the user's language (which is also exposed in other ways) and that the user wants captions or subtitles (which isn't).
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The tts:fontFamily attribute could expose the system's fonts and should use the same restrictions as CSS.
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The <audio> and <image> elements probably allow the server to detect the value of any <condition> expression. Many of the condition-functions seem to be already exposed by CSS media queries. The supports-functions probably don't expose any more than the UA string. So this may only be extra fingerprinting surface for UAs that aren't also general web browsers. However, if one of these functions exposes a user preference or device attribute, that would be extra fingerprinting surface.
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ttp:clockMode==local probably reveals the local time zone, if only by the timing of embedded resource requests. ttp:timeBase==clock reveals clock skew in the same way.
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If there's a way to pull out a display frame rate, that would also help fingerprinting.
If TTML is implemented natively in a user agent, it could expose fingerprinting vectors that aren't otherwise exposed. The spec should mention this risk so native implementations know to make intentional choices. The things I noticed to call out are:
Anything that's "implementation dependent" might be a fingerprinting vector.
<tts:color>and some others are described as implementation-dependent.A user's preference for how fast they consume media (e.g. 2x vs 2.5x).
The request for timed text indicates the user's language (which is also exposed in other ways) and that the user wants captions or subtitles (which isn't).
The
tts:fontFamilyattribute could expose the system's fonts and should use the same restrictions as CSS.The
<audio>and<image>elements probably allow the server to detect the value of any<condition>expression. Many of thecondition-functions seem to be already exposed by CSS media queries. Thesupports-functions probably don't expose any more than the UA string. So this may only be extra fingerprinting surface for UAs that aren't also general web browsers. However, if one of these functions exposes a user preference or device attribute, that would be extra fingerprinting surface.ttp:clockMode==localprobably reveals the local time zone, if only by the timing of embedded resource requests.ttp:timeBase==clockreveals clock skew in the same way.If there's a way to pull out a display frame rate, that would also help fingerprinting.