That is an example of a pun. It may not seem like one when you just read it, but try saying it out loud. If you pronounced it correctly, it should sound like the punchline is "To CSS html".

But what makes that a pun?

Well, the Oxford English Dictionary defines a pun as:

"The use of a word in such a way as to suggest two or more meanings or different
associations, or of two or more words of the same or nearly the same sound with
different meanings, so as to produce a humorous effect; a play on words."

So, this pun is made up of two parts:

The Setup:

Why did the chicken code a website in the sea?

The Punchline:

To see SS Aychtee Emel!

The setup brings two important factors to the front of your mind: Coding websites and the sea. This is important because the punchline deals with both of them. Written out, it tells you that the chicken wanted to see the Steam Ship named Aychtee Emel. However, when spoken out loud, it sounds as though the answer is "CSS html" which are two coding languages used to code websites. This is what is known as a play on words, and it is in a humorous context. Therefore, the joke above us a pun.

For more examples of puns broken down, click here.