src/main/java/j2html/TagCreator.java
public static String document(ContainerTag htmlTag) {
        if (htmlTag.getTagName().equals("html")) {
            return document().render() + htmlTag.render();
        }
        throw new IllegalArgumentException("Only HTML-tag can follow document declaration");
    }
public static String document(ContainerTag htmlTag) {
        if (htmlTag.getTagName().equals("html")) {
            return document().render() + htmlTag.render();
        }
        throw new IllegalArgumentException("Only HTML-tag can follow document declaration");
    }
Return a complete html document string

@param htmlTag the html content of a website
@return document declaration and rendered html content