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.directive('errorHref', function() {
    return {
        link: function(scope, element, attrs) {
            element.bind('error', function() {
                attrs.$set('ng-href', attrs.errorHref);
            });
        }
    }
});

<html ng-app="timeclock">
    <head ng-controller="headController">
        <link ng-href="{{urlToAppCSS}}" error-href="content/css/app.css" rel="stylesheet">

.directive('customStyle', function($http) {
    return {
        restrict: 'E',
        scope: {
            href: '@',
            fallback: '@'
        },
        link: function(scope) {
            $http.get(scope.href)
                    .then(function(response) {
                        // Take the contents of the response and place into
                        // a scope variable for use in the template
                        scope.css = response.data;
                    })
                    .catch(function(response) {
                        // The request failed, so instead try loading from scope.fallback url.
                    });
        },
        template: '<style>{{ scope.css }}</style>'
    }
});

<custom-style href="{{ urlToAppCSS }}" fallback="content/css/app.css"></custom-style>