code: function insert(sheet, [selectors, styles]) {
    if (sheet.scopedStyleId) {
      selectors = scopifySelectors(selectors, sheet.scopedStyleId);
    }
  
    var idx = sheet.insertRule(`${selectors} { }`, sheet.rules.length);
    var rule = sheet.rules[idx];
  
    if (typeof styles === 'string') { rule.style = styles; }
    else {
      // @TODO Fix this to be upward-friendly, and valueize style object.
      assign(rule.style, styles);
      //mirrorProperties(rule.style, styles);
    }
  
    return rule;
  }
docs: Insert a CSS rule, given by selector(s) and declarations, into a sheet. If the scoped attribute was specified, and scoping is not supported, then emulate scoping, by adding a data-* attribute to the parent element, and rewriting the selectors.