Source code

So that web servers and browsers work together smoothly worldwide, there are common standards. The most important for websites is HTML (Hypertext Markup Language), a markup language that structures content with so-called tags. The entire code that makes up a website or program is called source code.

Source code exists for websites as well as for computer programs. For websites, the browser reads the code and turns it into the visible page with content and functions. For software, there are two types: open-source software, whose source code is openly accessible and may be modified and redistributed (e.g. Linux or Shopware), and proprietary software, whose source code the manufacturer doesn't disclose (e.g. Microsoft Windows). One advantage of open source: many developers can find and fix bugs. The source code of websites, on the other hand, can in principle be viewed by anyone – at least the HTML, CSS and JavaScript code that the browser receives.

What is source code?

When you browse a website or social network, you rarely have problems reading the text on a page. It's clear, sometimes divided up for the eye by subheadings, bold text or coloured highlights. What we see, however, is based on certain instructions running in the background. These instructions are called source code.

Source code is written in a human-readable language and forms the basis of every website.

“Human-readable” means, in contrast to “machine-readable”, that the code consists of words and commands rather than pure numbers. Nevertheless, these languages have to be learned first. Code is written with code editors such as Visual Studio Code; in principle, however, any simple text editor will do. In everyday work, shop systems and CMS usually generate the source code automatically from templates.

Programming languages

There are countless programming languages. Which one is used depends on the purpose. Examples commonly found in the web environment include:

• PHP (e.g. Shopware, WordPress)

• Python

• JavaScript (for interaction in the browser and in frameworks such as Vue or React)

Markup languages

At their core, websites are based on the markup language HTML. It defines what content is: heading, paragraph, list, link, image. CSS determines what it all looks like; JavaScript provides interaction. Another markup language is XML, which is used for sitemaps and product data feeds, for example.

Viewing the source code of websites

Anyone can view the source code of a website. In Google Chrome, this is done by right-clicking and selecting “View page source” or with the shortcut Ctrl+U. This shows the HTML delivered by the server. Via “Inspect” (developer tools), on the other hand, you see the code as it looks in the browser after JavaScript has been executed. This difference matters for SEO, because Google doesn't always reliably capture content that is only loaded via JavaScript.

The importance of source code in search engine optimisation

In search engine optimisation, source code plays an important role, because search engines don't read the visible page but its code. If Googlebot can't process the code correctly, it can't index the page properly either. It's therefore important that relevant content, links and meta information are directly in the HTML or rendered reliably. Google processes the first 15 MB of an HTML file. Lean, clean code also speeds up loading time, which benefits users and Core Web Vitals.

The code should also mark up content semantically correctly: a clear heading hierarchy (H1, H2, H3), real links instead of click events, alt texts for images and structured data according to Schema.org. This helps search engines and AI systems understand what a page is about.

Attempts to deceive search engines via the source code – for example with hidden text or content that only bots see (cloaking) – violate Google's guidelines and can lead to penalties.