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The WP editor can do that sometimes, especially with javascript.

You could turn the button into a shortcode:

add_shortcode( 'flip_button', 'tu_flip_button' );
function tu_flip_button() {
    ob_start();
    ?>
    <button onclick="document.querySelector('#flip-toggle').classList.toggle('flip');"class="button transparent crni">Nazad</button>
    <?php
    $ret = ob_get_contents();
    ob_end_clean();
    return $ret;
}

Then you could do this:

[flip_button]

How to create shortcode? Like a script in footer hook or?

Sory for disturbing.

Thanks.

Add Tom's code using Code Snippet: https://docs.generatepress.com/article/adding-php/#code-snippets

Ok, thank you very much guys!

Best regards
Nikola

Glad we could help!