testRichInput.js

import { LightningElement } from 'lwc';

export default class TestRichInput extends LightningElement {
  value = `
    <table>
    <thead>
      <tr>
        <td>Header 1</td>
      <tr>
      <tr>
        <td>Header 2</td>
      <tr>
    </thead>
    <tbody>
      <tr>
        <td>Cell 1</td>
      <tr>
      <tr>
        <td>Cell 2</td>
      <tr>
    </tbody>
    </table>
  `

  getCaret(el) {
    if (el.selectionStart) {
      return el.selectionStart;
    } else if (document.selection) {
      el.focus();

      var r = document.selection.createRange();
      if (r == null) {
          return 0;
      }

      var re = el.createTextRange(),
          rc = re.duplicate();
      re.moveToBookmark(r.getBookmark());
      rc.setEndPoint('EndToStart', re);

      return rc.text.length;
    }
    return 0;
  }

  handleKeyDown(e) {
    const key = e.keyCode;
    const input = this.template.querySelector('c-custom-input');
    const pos = document.activeElement.selectionStart;
    const activeElement = document.activeElement;
    console.log(pos)
    console.log(activeElement)
    console.log(input.getContent())
    if (key === 8) {
      e.preventDefault();
      e.stopPropagation();
      input.setCustomContent(this.value);
    }
    this.value = input.getContent();
    console.log(this.value);
  }
}