Test me

[daKmoR]

The property backside is not doing anything.

See the demo below

import { html } from 'lit-html';
import 'demo-wc-card/demo-wc-card.js';

export const header = () => {
  return html`
    <demo-wc-card backside .header=${'my other header'}></demo-wc-card>
  `;
};

[daKmoR]

You probably meant to use the attribute back-side.

Here is it working

import { html } from 'lit-html';
import 'demo-wc-card/demo-wc-card.js';

export const header = () => {
  return html`
    <demo-wc-card back-side .header=${'my other header'}></demo-wc-card>
  `;
};