I don't think the real issue is that one. At least in Safari I get this:

    t = document.createElement("template")
    t.innerHTML = `<style id="outside"></style><svg><style id="inside"></style></svg>`
    t.content.querySelector("#outside").sheet // null
    t.content.querySelector("#inside").sheet // undefined

That means that the .sheet property is present on the style element, but not if it is inside an svg element.