[cssom] Missing step to set parentStyleSheet

[cdoublev]

In Chrome/Firefox, a CSSStyleRule added to CSSStyleSheet.cssRules by using CSSStyleSheet.insertRule(), will not have a parentStyleSheet:

The parentStyleSheet attribute must return the parent CSS style sheet.

Note: The only circumstance where null is returned when a rule has been removed.

Should it be ...or when a rule has been inserted?

Below is how I read an undefined parentStyleSheet for this new rule.

<style>
  div { color: red }
</style>
<script>
  const { styleSheets: [styleSheet] } = document
  const { cssRules } = styleSheet
  const [{ parentStyleSheet }] = cssRules
  console.log(styleSheet === parentStyleSheet)
  styleSheet.insertRule('p { color: red }')
  console.log(cssRules.length)
  const [, { parentStyleSheet2 }] = cssRules
  console.log((styleSheet === parentStyleSheet2) || parentStyleSheet2) // undefined
</script>

parent CSS style sheet: a reference to a parent CSS style sheet or null. This item is initialized to reference an associated style sheet when the rule is created. It can be changed to null.

It may be missing either in parse a CSS rule or insert CSS rule. I'm curious to learn the answer.

[cdoublev]

There was a typo in my code.