All CSS*Rule.style attributes except CSSFontFace.style are defined with [SameObject,PutForwards=cssText]:
Is it intentional?
The practical implication, in current Chrome/FF version:
const sheet = new CSSStyleSheet()
sheet.replaceSync('style {} @font-face {}')
const style = sheet.cssRules[0]
const font = sheet.cssRules[1]
style.style = 'color: green'
style.cssText; // style { color: green }
font.style = 'font-style: italic'
font.cssText; // @font-face {}