How do I draw an Ideal line between two headers?

My HTML code has this structure:

h1 {
  text-align: center;
  font-size: 20pt;
  padding-bottom: 0;
}

h2 {
  text-align: center;
  font-size: 16pt;
}

<body>
  ...
  <div id="container">
    <h1>Title</h1>
    <h2>Sub</h2>
  </div>
  ...
</body>

I want to "draw" a line between those two headings like this:

It should adjust to the width of the headings:

(Excuse my image editing skills)

Is there a simple css-only way to do so?

Solution

You can handle this problem with a simply pseudo element. No html structure changing or non-support css of cross browers. We need to use table display on the parent and make a new element (your border with pseudo) on the <h1>.

Look at this example:

#container {
  display: table;
  margin: 0 auto;
  text-align: center;
}

h1:after {
  content:"";
  position: absolute;
  bottom: 0;
  left: 0;
  width: 100%;
  border-bottom: 3px solid red;
}

h1, h2 {
  margin: 5px 0;
  padding: 0 10px 5px;
}

h1 {
  position: relative;
  font-size: 20pt;
}

h2 {
  font-size: 16pt;
}

  <div id="container">
    <h1>long long title</h1>
    <h2>Sub</h2>
  </div>

  <div id="container">
    <h1>h1</h1>
    <h2>Sub</h2>
  </div>


 <div id="container">
    <h1>h1</h1>
    <h2>long long subtitle</h2>
  </div>

Fiddle demo