JSFiddle - React, Tailwind, and code Playground

HTML

<div class="content" >
    --- <a class="b" href="/?q=spoiler">spoiler</a> ---
    <br>
    <br>
    dangerous text here
    <br>
    --- <a class="b" href="/?q=spoiler">spoiler</a> ---
    <br>
    safe text here
    <br>
    --- <a class="b" href="/?q=spoiler">spoiler</a> ---
    <br>
    <br>
    dangerous text here
    <br>
    --- <a class="b" href="/?q=spoiler">spoiler</a> ---    
</div>

JavaScript

$('.content a:contains("spoiler").b:even').each(function(){
    $(this).nextUntil('.content a:contains("spoiler").b')
           .wrapAll('<div style="border:solid 1px black;"></div>');
});