The StyleSheetList interface represents a list of StyleSheet.
It is an array-like object but can't be iterated over using Array methods. However It can be iterated over in a standard for loop over its indices, or converted to an Array.
Examples
Get document styleSheet objects with for loop
for (var i=0; i < document.styleSheets.length; i++){
  var styleSheet = document.styleSheets[i];
}
Get all CSS rules for the document using Array methods
var allCSS = 
    [].slice.call(document.styleSheets)
        .reduce(function (prev, styleSheet) {
            if (styleSheet.cssRules) {
                return prev +
                    [].slice.call(styleSheet.cssRules)
                        .reduce(function (prev, cssRule) {
                            return prev + cssRule.cssText;
                        }, '');
            } else {
                return prev;
            }
        }, '');
Specifications
| Specification | Status | Comment | 
|---|---|---|
| CSS Object Model (CSSOM) The definition of 'CSSStyleSheetList' in that specification. | Working Draft |