According to https://stackoverflow.com/a/47309461, -webkit-baseline-middle is defined in Chrome and -moz-middle-with-baseline is defined in Firefox to align the vertical midpoint of the box with the baseline of the parent box plus half the x-height of the parent, as was defined in CSS 2.1.

It seems that these values are used by these browsers in their internal style-sheets when the deprecated attribute align is set to middle for an embed element.

The spec doesn't mention these values, but considering their use across multiple browsers, I'm wondering if it makes sense to actually define them anyway. I know browser engines usually follow the spec, but it's not unprecedented for it to go the other way. I just feel like it may be good to define these without browser prefixes, so bugs can be open with the relevant browsers to remove the prefixes, as browsers are moving away from prefixed values.