src/main/java/j2html/tags/Tag.java
public T attr(String attribute, Object value) {
        setAttribute(attribute, value == null ? null : String.valueOf(value));
        return (T) this;
    }
public T attr(String attribute, Object value) {
        setAttribute(attribute, value == null ? null : String.valueOf(value));
        return (T) this;
    }
Sets a custom attribute

@param attribute the attribute name
@param value     the attribute value
@return itself for easy chaining