Forbid multiple and unexpected CSS style declarations

Related issue:
- uBlockOrigin/uBlock-issues#1806 (comment)

src/js/static-filtering-parser.js

@@ -1437,12 +1437,17 @@ Parser.prototype.SelectorCompiler = class {
  // https://github.com/uBlockOrigin/uBlock-issues/issues/1751
  // Do not rely on matches() or querySelector() to test whether a
  // selector is declarative or not.
+ // https://github.com/uBlockOrigin/uBlock-issues/issues/1806#issuecomment-963278382
+ // Forbid multiple and unexpected CSS style declarations.
  sheetSelectable(s) {
  if ( this.reSimpleSelector.test(s) ) { return true; }
  if ( this.stylesheet === null ) { return true; }
  try {
  this.stylesheet.insertRule(`${s}{color:red}`);
- if ( this.stylesheet.cssRules.length === 0 ) { return false; }
+ if ( this.stylesheet.cssRules.length !== 1 ) { return false; }
+ const style = this.stylesheet.cssRules[0].style;
+ if ( style.length !== 1 ) { return false; }
+ if ( style.getPropertyValue('color') !== 'red' ) { return false; }
  this.stylesheet.deleteRule(0);
  } catch (ex) {
  return false;