Browsers and servers

A browser is an HTTP client. You give it a URL, and it makes a request on your behalf.

The server receives that request and decides what to send back. It might read an HTML file from disk. It might generate HTML using data from a database. The browser does not need to know which approach the server used.

A simplified exchange looks like this:

GET /about/ HTTP/1.1
Host: example.com

The server could answer:

HTTP/1.1 200 OK
Content-Type: text/html

<h1>About us</h1>

The browser reads the Content-Type header and knows the response contains HTML. It parses the markup and turns it into a document on the screen.

This distinction is useful:

• the server sends HTML
• the browser interprets HTML

The browser also provides default styles. This is why a heading appears larger than a paragraph even before you write any CSS.

HTML is forgiving. Browsers try hard to display imperfect documents. That helps the Web keep working, but it also means a page that looks correct can still contain mistakes. Later we’ll use validation tools to find them.