Stylesheet switcher sans ES6 features

switcher-es5.js

(function() {
  window.addEventListener( 'load', function() {
    var links = Array.prototype.slice.call( document.querySelectorAll( 'link[title]') );
    var uniqueTitles = links.reduce( function( titles, link ) {
      if ( titles.indexOf( link.title ) === -1 ) {
        titles.push( link.title );
      }
      return titles;
    }, []);
    var title = window.sessionStorage && window.sessionStorage.getItem( 'title' ) || '';
    var select;
    var option;
    var i = 0;

    function selectStyle( title ) {
      links.forEach( function( link ) {
        link.disabled = true;
        link.disabled = link.title !== title;
      });

      if ( window.sessionStorage ) {
        window.sessionStorage.setItem( 'title', title );
      }
    }

    if ( uniqueTitles.length > 1 ) {
      if ( title ) {
        selectStyle( title );
      }
      select = document.createElement( 'select' );
      select.setAttribute( 'style', 'position: absolute; top: 1em; right: 1em;' );
      for ( ; i < uniqueTitles.length; i++ ) {
        option = document.createElement( 'option' );
        option.text = 'CSS: ' + uniqueTitles[ i ];
        option.value = uniqueTitles[ i ];
        option.selected = uniqueTitles[ i ] === title;
        select.add( option );
      }
      document.body.appendChild( select );

      select.addEventListener( 'change', function( event ) {
        title = event.target.options[ event.target.selectedIndex ].value; 
        selectStyle( title );
      });
    }
  });
}());

sessionStorage is probably good enough

Wait for the document to have fully loaded so that all stylesheets are available