Why is textarea filled with mysterious white spaces?

Question

I have a simple text area in a form like this:

    <?php if(         
<script async src="https://pagead2.googlesyndication.com/pagead/js/adsbygoogle.js"></script>
<ins class="adsbygoogle"
     style="display:block"
     data-ad-client="ca-pub-5408099190056760"
     data-ad-slot="7305827575"
     data-ad-format="auto"
     data-full-width-responsive="true"></ins>
<script>
     (adsbygoogle = window.adsbygoogle || []).push({});
</script>        </div>
      </div>
      
      <div class="fly-panel detail-box" id="flyReply">
        <fieldset class="layui-elem-field layui-field-title" style="text-align: center;">
          <legend>21条回答</legend>        </fieldset>

        <ul class="jieda" id="jieda">
                    <li data-id="111" class="jieda-daan">
            <a name="item-1111111111"></a>
            <div class="detail-about detail-about-reply">
                         <a class="fly-avatar" href="">
                <img src="https://www.e-learn.cn/qa/data/avatar/000/00/00/small_000000077.jpg" alt=" 粉色の甜心 ">
              </a>
              <div class="fly-detail-user">
                <a href="" class="fly-link">
                  <cite> 粉色の甜心</cite>
                                             
                </a>
                
                <span>(楼主)</span>
            
              </div>              <div class="detail-hits">
                <span>2020-11-28 02:31</span>
              </div>

            </div>
            <div class="detail-body jieda-body photos">
              <p>          
<p>I got the same problem, and the solution is very simple: <strong>don't start a new line!</strong> Although some of the previous answers can solve the problem, the idea is not stated clearly. 
The <strong>important understanding is, to get rid of the unintended spaces, never start a new line just after your start tag.</strong> </p>

<p><strong>The following way is WRONG</strong> and will leave a lot of unwanted spaces before your text content:</p>

<pre><code> <textarea>
    text content // start with a new line will leave a lot of unwanted spaces
 

The RIGHT WAY to do it is:

 text content //put text content right after your start tag, no new line