src/main/java/j2html/tags/Tag.java
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