title: Can't get tag's content with xpath
body: <p>I'm trying to get the 'Shipping Weight' value(i.e 4.65 pounds) from amazon page, here is the part involved(from <a href="https://rads.stackoverflow.com/amzn/click/com/B0018RSEMU" rel="nofollow noreferrer" rel="nofollow noreferrer">https://www.amazon.com/dp/B0018RSEMU</a>):</p>
  
  <pre><code>&lt;div class="a-row a-spacing-top-base"&gt;  
    &lt;div class="a-column a-span6"&gt;  
      &lt;div class="a-row a-spacing-base"&gt;  
  
          &lt;div class="a-section table-padding"&gt;  
            &lt;table id="productDetails_detailBullets_sections1" class="a-keyvalue prodDetTable" role="presentation"&gt;  
  
                        &lt;tr&gt;  
                            &lt;th class="a-color-secondary a-size-base prodDetSectionEntry"&gt;  
                            Shipping Weight  
                            &lt;/th&gt;  
  
                            &lt;td class="a-size-base"&gt;   
                          4.65 pounds (&lt;a href="https://www.amazon.com/gp/help/seller/shipping.html/ref=dp_pd_shipping?_encoding=UTF8&amp;amp;seller=ATVPDKIKX0DER&amp;amp;asin=B0018RSEMU"&gt;View shipping rates and   policies&lt;/a&gt;)  
                          &lt;/td&gt;  
                          &lt;/tr&gt;                            
                          ......
  </code></pre>
  
  <p>I code like this:  </p>
  
  <pre><code>from lxml import html   
  import requests  
  headers = {'User-Agent': '...'}  
  page = requests.get(url, headers = headers)  
  
  doc = html.fromstring(page.content)  
  XPATH_WEIGHT = '//th[contains(text(),"Shipping Weight")]/following-sibling::td/text()'  
  RAW_WEIGHT = doc.xpath(XPATH_WEIGHT)  
  </code></pre>
  
  <p>after running, it returns nothing. What's the problem? With the same syntax, I can get other tag's text correctly. so confused here.</p>