An HTML document is like a big family tree, with parents, siblings, children, ancestors, and descendants.

It comes from the ability to nest HTML elements within one another.

Nesting

Let’s write a simple paragraph and enhance it by differentiating parts of the text, by inserting two inline elements:

<p>
  Sir <strong>Alex Ferguson</strong> once said about Filipo Inzaghi:<q>"That lad must have been born offside."</q>.
</p>

In this setup:

• the <strong> element gives the words “Alex Ferguson” more importance
• the <q> marks his quote about Inzaghi

The fact that <strong> is displayed in bold is only the browser’s default behavior. Remember that you have to choose HTML elements according to their meaning, not how they look like.

In this case:

• <p> is the parent element of <strong> and <q>
• <strong> and <q> are child elements of <p>
• <strong> and <q> are sibling elements

Order

How nesting works depends on the location of opening and closing tags.

Because an HTML element comprises an opening tag, a closing tag, and everything in between, a child element must be closed before closing the parent element.

<!-- This is INVALID code! :-( -->
<p>
  This HTML code won't work because I the "strong" tag is opened here <strong>but is only closed after the paragraph.
</p></strong>

Because the <strong> was opened after the <p> (and is thus considered a child of <p>), the <strong> element must be closed before its parent <p>.

<!-- This is valid code. :-) -->
<p>
  This HTML code will work because I the "strong" tag is opened <strong>and closed</strong> properly.
</p>

Depth

Because child elements can themselves contain other child elements, it’s possible to write a deeper hierarchy within an HTML document.

Our above paragraph could be part of a blog article:

<article>
  <h1>Famous football quotes</h1>
  <p>
    Sir <strong>Alex Ferguson</strong> once said about Filipo Inzaghi:<q>"That lad must have been born offside"</q>.
  </p>
  <p>
    When criticized by John Carew, <strong>Zlatan Ibrahimovic</strong> replied: <q>"What Carew does with a football, I can do with an orange"</q>.
  </p>
  <p>
    <strong>George Best</strong> said <q>"I spent a lot of money on booze, birds and fast cars. The rest I just squandered."</q> when asked about his lifestyle.
  </p>
</article>

In this setup:

• <article> is the ancestor of every other element
• <article> is the parent of the <h1> and the 3 <p>
• <h1> and the 3 <p> are siblings
• each <p> is the parent of the <strong> and <q> they contain
• every <h1>, <p>, <strong> and <q> are all descendants of <article>

The family tree analogy still applies when traversing several layers of HTML nesting:

• a descendant of element X is any element contained within X
• a child is just a direct descendant
• an ancestor of element Y is any element that contains Y
• the parent is just the first direct ancestor
• a sibling of element X is any element which has the same parent

Block and inline nesting

Block elements can contain either block or inline elements.

However, inline elements can only contain other inline elements ¹.

<!-- This is INVALID code! :-( -->
<strong>
  <p>You can't put a paragraph inside a "strong" tag.
</strong>

Just remember ancestor/descendant, parent/child, and siblings. This hierarchy will be useful in CSS.