* Read the HTML specification for Custom Element Reactions: {@link https://html.spec.whatwg.org/dev/custom-elements.html#custom-element-reactions|Custom Element Reactions}.
*
* @startuml custom-control.png
* skinparam monochrome true
* skinparam shadowing false
* HTMLElement <|-- CustomElement
* CustomElement <|-- CustomControl
* @enduml
* @summary A base class for custom controls based on CustomElement.