Custom Component using HTMLElement

I is a boy.

Log

(Apr-6) Uncaught Syntax Error: mistake is not a valid custom name.

(Apr-6) Uncaught Syntax Error: mistakeTag is not a valid custom name.

(Apr-6) custom tag name must have hyphen (-)

(Apr-6) text-decoration of CSS doesn't effect if the property has been defined in custom element class defined.

(Apr-6) text-decoration of the element will effect on its pseudo element, even if the property is defined under the pseudo element.

(Apr-6) The line-through problem is solved by abandoning pseudo element after, and switching to insertBefore(createElement,nextSibling). Because I have not removed event listener, the correct answer can be added infinitely. If I want to alternate between displaying and not displaying correct answer, I'll need a second click handler and alternate their listening.

test custom element 3 insertBefore nextSibling