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The benefits and limitations of headless browser testing

Headless browsers offer efficient web UI testing but lack the visual debugging capabilities of traditional browsers, making assessing the app's state upon test failure challenging.

Headless browsers are a great way to speed up tests and gain more efficiency out of the hardware that tests run on, but they might not be ideal for debugging failing tests or tests where you'd want to see a screenshot in the case of a failure.

With recent advancements in headless browser design, there are fewer differences between the way a headless browser renders a webpage's content compared to a traditional browser.

Headless browser testing is essentially web UI testing, but it uses tools that let tests run without loading a complete web browser. These tools send requests the same way a normal web browser does and receive the same HTML, CSS and JavaScript files back.

The key difference is that they don't render these files in a GUI. Instead, the headless browser builds an internal model for the webpage and lets tests interact with objects on the page -- the same way they would with a normal web browser, all without the additional overhead of a GUI.

Advantages of headless browser testing

There are major benefits of headless browser testing, including the following:

  • Faster test runs. The biggest motivation to choose headless browsers over testing with a traditional browser is that headless browsers shorten the time it takes to run a test suite. Headless browsers can run automated UI tests faster because they don't render UI elements in a graphical environment. The time required to display something in your browser is significant; by eliminating that step, a headless browser can run tests quicker than a traditional web browser.
  • Increased hardware efficiency. Because headless browsers don't render UI elements, they run more efficiently than traditional browsers and can, therefore, increase the efficiency of the hardware you use to run tests. Similarly, running tests with a headless browser can often significantly increase the number of tests you can run in parallel because the load per test on the hardware is lower. More tests running in parallel means less time overall running your test suite.
  • Easier-to-run tests in a CI/CD pipeline. Another benefit of running tests with headless browsers is the ease of running tests in a CI/CD pipeline. Often, your pipeline runs on a remote machine, where it isn't always possible to launch a traditional browser. If you're already running headless tests locally, transitioning these to run on a remote machine is often seamless and simple.

Disadvantages of headless browser testing

There are some limitations with headless browser testing, including the following:

  • Unpredictable browser behavior. The tradeoff of skipping the step to render elements in a UI is that headless browsers don't always behave the same way traditional web browsers do. Even when comparing one traditional web browser to another, the webpage can behave differently in each. The reason for this difference in behavior is subtle differences in the way that browsers render UI elements generated from a webpage's HTML, CSS and JavaScript content. There are, of course, high-level standards for how web browsers should render content, but just in the same way that a Word document saved in Microsoft Word might look different if opened in Google Docs, there can be minor unexpected differences.
  • Might not be suited to visual elements. In the past, it was true that headless browser testing was not well suited for tests that are focused on visual elements of an application. But headless browsers have come a long way, and a 2023 update to Google Chrome's headless browser has merged the code paths of headless and headful mode, where the only difference between the two modes is that, in headless mode, Chrome creates but doesn't display any browser windows. Certainly, testers must validate these claims for themselves, but it's a huge step forward for headless browser functionality and further contributes toward the advantages of using headless browsers for testing.
  • Debugging can be difficult. Most tests that run with a traditional browser provide a screenshot when a test fails. That makes it easy to observe the visual state of the application at the time of the failure. With headless browsers, there's no screenshot capability, so you must read over an error message telling you which selector couldn't be located. Without the bigger picture, it's difficult to glean an understanding of a test failure. In this case, you can resort to the functionality built into headless browsers that lets you print out a page's HTML source to fully understand the state of an application.

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