snappymail/vendors/knockout/src/binding/defaultBindings/attr.js

var attrHtmlToJavaScriptMap = { 'class': 'className', 'for': 'htmlFor' };
ko.bindingHandlers['attr'] = {
    'update': function(element, valueAccessor, allBindings) {
        var value = ko.utils.unwrapObservable(valueAccessor()) || {};
        ko.utils.objectForEach(value, function(attrName, attrValue) {
            attrValue = ko.utils.unwrapObservable(attrValue);

            // Find the namespace of this attribute, if any.
            var prefixLen = attrName.indexOf(':');
            var namespace = "lookupNamespaceURI" in element && prefixLen > 0 && element.lookupNamespaceURI(attrName.substr(0, prefixLen));

            // To cover cases like "attr: { checked:someProp }", we want to remove the attribute entirely
            // when someProp is a "no value"-like value (strictly null, false, or undefined)
            // (because the absence of the "checked" attr is how to mark an element as not checked, etc.)
            var toRemove = (attrValue === false) || (attrValue === null) || (attrValue === undefined);
            if (toRemove) {
                namespace ? element.removeAttributeNS(namespace, attrName) : element.removeAttribute(attrName);
            } else {
                attrValue = attrValue.toString();
            }

            // In IE <= 7 and IE8 Quirks Mode, you have to use the JavaScript property name instead of the
            // HTML attribute name for certain attributes. IE8 Standards Mode supports the correct behavior,
            // but instead of figuring out the mode, we'll just set the attribute through the JavaScript
            // property for IE <= 8.
            if (ko.utils.ieVersion <= 8 && attrName in attrHtmlToJavaScriptMap) {
                attrName = attrHtmlToJavaScriptMap[attrName];
                if (toRemove)
                    element.removeAttribute(attrName);
                else
                    element[attrName] = attrValue;
            } else if (!toRemove) {
                namespace ? element.setAttributeNS(namespace, attrName, attrValue) : element.setAttribute(attrName, attrValue);
            }

            // Treat "name" specially - although you can think of it as an attribute, it also needs
            // special handling on older versions of IE (https://github.com/SteveSanderson/knockout/pull/333)
            // Deliberately being case-sensitive here because XHTML would regard "Name" as a different thing
            // entirely, and there's no strong reason to allow for such casing in HTML.
            if (attrName === "name") {
                ko.utils.setElementName(element, toRemove ? "" : attrValue);
            }
        });
    }
};