Attributes in custom element constructor missing dependent on script tag location

I try to get a html custom element to work:

When viewing the following example in the browser I must conclude that this.dataset.text attribute in the constructor() is:

present before the script defining the custom element
empty after the script defining the custom element

<html>
    <head>
        <title>myimage</title>
        <meta charset="UTF-8">
        <meta name="viewport" content="width=device-width, initial-scale=1.0">
    </head>
    <body>
        <div>
        before custom element definition: <my-element data-text="Replaced and works as expected">Not Replaced</my-element>
        </div>
        <script>
          class MyElement extends HTMLElement {
            constructor() {
              super();
              this.text_attribute= this.dataset.text;
            }

            connectedCallback() {
              if (this.text_attribute) {
                this.innerText = this.text_attribute;
              }
            }
          }
          customElements.define('my-element', MyElement);
        </script>
        <div>
        after custom element definition: <my-element data-text="Replaced">Not Replaced</my-element>
        </div>
    </body>
</html>

This seems to be a common behavior in chrome and Firefox. I can provide some more facts:

the this.dataset.text is always present in connectedCallback()
accessing the text-attribut via this.getAttribute('data-text') does behave in the same way as above

Can anyone explain the purpose of this seemingly buggy behavior?

Solution

This isn't buggy behavior.

Your first <my-element> is processed by the DOM parser before you define it.
Thus all attributes are known (in the DOM) when the constructor executes.

In general you don't access DOM in the constructor; as it also runs when you do:

 let foo = document.createElement("my-element"); // absolutly no DOM created

Here is a trimmed down example:

<style>
  my-element { display:block }
</style>
<my-element id="FOO"> ONE </my-element>
<script>
  customElements.define('my-element', class extends HTMLElement {
    constructor() {
      super();
      document.body.append(` ► constructor id:${this.id} ◄ `);
    }
    connectedCallback() {
      document.body.append(` ► connected id:${this.id} ◄ `);
    }
  });
</script>
<my-element id="BAR"> TWO </my-element>
<my-element id="BAZ"> THREE </my-element>

NOTE

How FOO innerHTML is written (because it was parsed) to the DOM before its constructor runs
How BAR and BAZ constructor run before its innerHTML is written to the DOM
and the id value doesn't exist yet
I have seen many courses where they "save" you from this standard behavior by executing the <script> async, thus after all DOM is parsed. They don't understand the technology...