It’s easy to measure element size. And I was surprised when I ran my code in Opera and got 0 of the button height.
I thought by the time, when DOMContentLoaded event fired, DOM construction had been compliteed and all elements had obtained its sizes.

HTML

<button id="b" class="btn btn-primary" type="button">...</button>

JS

document.addEventListener('DOMContentLoaded', (ev) => {
    let b = document.querySelector('#b');
    let h = b.offsetHeight; // 37 (Edge), 0 (Opera)
});

This code works fine in Edge browser but Opera seems to have another behavior and doesn't wait for DOM to be created. So I was forced to make artificial delay. But it looks ugly enought.

document.addEventListener('DOMContentLoaded', async (ev) => {
    await new Promise((resolve, reject) => {setTimeout(() => resolve(), 200)});
    // ...
});

Is there common way to solve suck a prbnlem?
Thanks.