HTML custom tags (HTML functions)
<p>Is it possible to create HTML functions like you can do with... I don't know... JS?</p>

<p>Could you do something like this?</p>

<pre><code>&lt;!DOCTYPE html&gt;
  &lt;html version=5&gt;
    &lt;body&gt;
      &lt;function name="para" attr1="text"&gt;
        &lt;p localName="para"&gt;&lt;/p&gt;
        &lt;script type="functionHTML/javaScript"&gt;
          function.data("attr1")includes = getLocalElementByName("para")
        &lt;/script&gt;
      &lt;/function&gt;
      &lt;-para text="Hello World!"&gt;&lt;/-para&gt;
    &lt;/body&gt;
  &lt;/html&gt;
</code></pre>

<p>This would come out on the webpage as:</p>

<p>Hello World!</p>