public T withClasses(String... classes) {
        StringBuilder sb = new StringBuilder();
        for (String s : classes) {
            sb.append(s != null ? s : "").append(" ");
        }
        return attr(Attr.CLASS, sb.toString().trim());
    }
public T withClasses(String... classes) {
        StringBuilder sb = new StringBuilder();
        for (String s : classes) {
            sb.append(s != null ? s : "").append(" ");
        }
        return attr(Attr.CLASS, sb.toString().trim());
    }
public T withClasses ( String ... classes ) { StringBuilder sb = new StringBuilder ( ) ; for ( String s : classes ) { sb . append ( s != null ? s : "" ) . append ( " " ) ; } return attr ( Attr . CLASS , sb . toString ( ) . trim ( ) ) ; }
Convenience methods that call attr with predefined attributes

@return itself for easy chaining
Convenience methods that call attr with predefined attributes