src/main/java/j2html/tags/ContainerTag.java
public ContainerTag with(DomContent child) {
        if (this == child) {
            throw new RuntimeException("Cannot append a tag to itself.");
        }
        if (child == null) {
            return this; // in some cases, like when using iff(), we ignore null children
        }
        children.add(child);
        return this;
    }
public ContainerTag with(DomContent child) {
        if (this == child) {
            throw new RuntimeException("Cannot append a tag to itself.");
        }
        if (child == null) {
            return this; // in some cases, like when using iff(), we ignore null children
        }
        children.add(child);
        return this;
    }
Appends a DomContent-object to the end of this element

@param child DomContent-object to be appended
@return itself for easy chaining