currently I am using flatsome theme to build a website and I have to make it work with Accessibility model WCAG 2.0 (Level AA). So, while I passed the check in AChecker. In Wave I have errors like Empty button.

I have already used this function in my php:

function start_modify_html()
{
    ob_start();
}
function end_modify_html()
{
    $html = ob_get_clean();
    $html = str_replace('<i class="icon-', '<span class="icon-', $html);
    .
    .
    .
    .
    echo $html;
}
add_action('wp_head', 'start_modify_html');
add_action('wp_footer', 'end_modify_html');

But the errors are below my footer, because it has to do with mobile menu:

<button class="toggle">
<i class="icon-angle-down"></i>
</button>

and as you can see, even the <i> hasn't changed to <span>. I have already searched and try a few things but nothing worked.

Has anyone of you know how to overcome this? I have even tried to found it in the php files and I wasn't lucky.