See the following code: <!DOCTYPE html> <p style=""></p> <script> let p = document.querySelector('p'); alert(getComputedStyle(p).maskOrigin); p.style.mask = "none"; alert(getComputedStyle(p).maskOrigin); </script> The initial value of mask-origin property is "border-box", however, setting mask shorthand resets the value to its non-initial value "content-box" unexpectedly. It is probably because StyleGeometryBox::ContentBox corresponds to zero.
Opened a PR in Servo: https://github.com/servo/servo/pull/15908 Their initial values were wrong. Also contains other wrong single keyword properties. Also found out that mask-repeat's initial value is wrong in gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=1346193
Pushed by firstname.lastname@example.org: https://hg.mozilla.org/integration/autoland/rev/9da96b87d014 Adjust stylo expectations for test_computed_style.html a=bustage