Steps to Reproduce:
Create a SCSS declaration with:

.some-class {
	inset-block-end: 0;
}

Actual Results:

.some-class {
	inset-block-end: 0;
}

Expected Results:

[dir=ltr] .some-class {
	left: 0;
}
[dir=rtl] .some-class {
	right: 0;
}
.some-class {
	inset-inline-start: 0;
}

Firefox and Chrome understand inset-inline-start, but Safari and IE11 don't. So things look broken there. This is currently affecting the Popover component.