What Is a Headless Browser and Where Is It Used?

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A headless browser is a web browser without a graphical user interface (GUI). Unlike regular browsers like Chrome and Firefox that display web pages visually, a headless browser can browse the web programmatically without rendering the page visually. Headless browsers provide all the standard web browsing capabilities, including executing JavaScript, handling cookies, managing storage, and adaptively rendering CSS, but do so without a user interface.

Headless browsers offer a number of benefits:

Automated Testing

Headless browsers are commonly used for automated testing of web pages and applications. They provide an automated way to interact with web content, submitting forms, clicking page elements, and validating results. Automated headless browser testing is faster and more efficient than driving tests through a full browser GUI.

For example, a headless browser can rapidly fill out and submit forms on a webpage to verify successful form handling. Tests that involve navigating menus, clicking buttons, and validating page text can all be automated via headless browser scripts. These kind of automation tests are much faster to execute in a headless browser compared to a full GUI.

Headless browsers are also useful for cross-browser testing. Scripts can run parallel tests across Firefox, Chrome, and others in headless mode to validate consistency across browsers with minimal overhead.

Web Scraping and Crawling

Headless browsers are invaluable for web scraping and crawling websites to extract data. Many websites rely heavily on JavaScript to render content, which can be difficult for tools like cURL to handle properly. Headless browsers can execute JavaScript and render full pages, allowing easy extraction of complex dynamic content.

For example, a site may dynamically load content via AJAX requests, or render page data on the client-side with JavaScript. A headless browser can crawl these sites and wait for all content to fully load before scraping the final rendered HTML. Libraries like Puppeteer provide APIs that make it easy to extract scraped content from a headless browser.

Headless browsers are also effective at crawling Single Page Apps (SPAs) that extensively use client-side JavaScript to update sections of a page. By evaluating the JavaScript, the full rendered app can be scraped.

Site Rendering

Since headless browsers can render full web pages, they can be used to generate static HTML versions of sites, take screenshots of pages, convert pages to PDF, and perform other rendering tasks. These can be scripted without the overhead of launching a full browser.

For example, a headless browser could be used to programmatically scroll through a long web page and take screenshots at regular intervals. These screenshots could then be stitched together to create a full page capture. For converting sites to PDF, the headless browser renders the full web page then converts the result to a PDF file.

Capturing rendered pages at various viewport sizes is also possible using a headless browser. This can aid responsive design testing by generating screenshots for mobile, tablet, and desktop renditions of the site from the same codebase.

Performance Testing

Testing website performance in a headless browser provides more accurate results than using a full browser. The interface and visual rendering consume significant resources, so testing in headless mode eliminates that overhead.

Performance testing in a headless browser focuses on page load times, runtime evaluation of JavaScript, and network utilization. Detailed Chrome DevTools integrations even allow tracking detailed performance metrics like JS heap size and DOM node count in headless runs.

Running load testing scripts in headless browsers can simulate thousands of concurrent users efficiently. The automated interactions drive load on the backend without the memory overhead of graphical browser sessions.

A Brief History of Headless Browsers

The concept of headless browsing first emerged in the early 2000s as browsers became increasingly complex. Loading an entire traditional browser UI required large amounts of memory and compute resources. Developers needed ways to automate browser tasks without the bloat of a GUI.

Some of the earliest headless browsers were HTMLUnit and PhantomJS. HTMLUnit, written in Java, was one of the first dedicated headless browsers optimized for automation tasks like testing. PhantomJS provided a scriptable headless browser based on WebKit.

When Google Chrome became popular, developers quickly realized the potential of a Chrome-based headless browser. In 2017, the Chrome team officially released Headless Chrome which enabled running Chrome in headless mode.

In 2018, Firefox followed suit by adding support for Headless Firefox. Now two of the most widely used browsers offered integrated headless functionality.

The evolution of developer tools like Puppeteer and Playwright have made orchestrating headless browsers even easier. They provide high-level APIs for automating actions in code. As headless testing and scraping become more mainstream, the tools continue improving.

How Headless Browsers Work

Under the hood, headless browsers work much like a traditional browser, but without rendering a visual interface. Here are the basic steps:

  1. The headless browser parses requested URLs just like a normal browser. It evaluates the HTTP response code, MIME type, cookies, and other headers.

  2. It builds the parsed DOM tree from the raw HTML while evaluating any CSS rules applied to elements. Style computations happen without painting anything to screen.

  3. Any JavaScript on the page executes to manipulate the DOM, make network requests, or retrieve data. The headless browser runs the JS against the in-memory DOM.

  4. The resulting post-JavaScript DOM tree contains all visible rendered content, computed styles, and evaluated visual dimensions.

  5. Instead of painting this to a screen, the headless browser can return the DOM directly or take actions like taking a screenshot, converting to PDF, or extracting data.

So essentially, a headless browser does everything an interactive browser would do except the final rendering and GUI steps. This provides major performance benefits.

Under the hood, popular headless browsers like Headless Chrome utilize the Blink rendering engine. Headless Firefox uses Gecko/Quantum. The engine differences affect CSS support, standards conformance, and execution performance.

Major Headless Browser Options

There are a few major tools available for headless browsing, each with their own strengths and weaknesses:

Headless Chrome

  • Google Chrome with the --headless flag enabled. Provides full Chrome rendering without the interface.

  • Very fast and stable since it‘s based on Chrome‘s rock-solid Blink engine.

  • Can feel bloated at times compared to leaner headless browsers.

  • Full support for modern web standards and ES6 JavaScript features.

  • Excellent DevTools integration allows tracking detailed performance metrics of headless tests.

Headless Firefox

  • Firefox also provides a headless mode via the --headless flag. Offers Firefox-flavored browsing.

  • Based on Gecko engine, so standards support differs slightly from Blink-based browsers.

  • Lighter weight than Chrome, but not as fast in benchmarks.

  • Supports more niche browser features thanks to Mozilla‘s focus on privacy.

  • Can be used as an occasional sanity check on Chrome-centric headless tests.

HTMLUnit

  • A veteran headless browser written in Java, optimized for automation tasks.

  • Very lightweight and fast since it‘s not based on the bloat of a mainstream browser‘s engine.

  • Lacks support for some modern web standards and JS environments.

  • Java-based APIs allows tight integration into JVM ecosystems.

  • Widely used for web automation testing thanks to early origins.

PhantomJS

  • A once popular headless browser based on WebKit, now discontinued.

  • Much slower than current options since it‘s based on older WebKit builds.

  • Lacks support for ES6, modern standards, and security updates.

  • Still used by some legacy systems, but not recommended for new projects.

  • Paved the way for many modern headless browser options and techniques.

Headless Browser Scripting

All the major JavaScript frameworks make it easy to drive headless browsers for testing and automation:

Puppeteer (Node.js)

// Print the user agent string to console
const puppeteer = require(‘puppeteer‘);

(async () => {
  const browser = await puppeteer.launch();
  const page = await browser.newPage();
  const userAgent = await page.evaluate(() => navigator.userAgent);
  console.log(userAgent);
  await browser.close();
})();

Playwright (Node.js, Python, .NET)

from playwright.sync_api import sync_playwright

with sync_playwright() as p:
    browser = p.chromium.launch(headless=True)
    page = browser.new_page()
    page.goto("http://whatsmyuseragent.org/")
    user_agent = page.inner_text(‘.user-agent‘)
    print(user_agent)
    browser.close()

Selenium (Java, Python, C#, etc)

WebDriver driver = new ChromeDriver(options.setHeadless(true)); 

driver.get("http://whatsmyuseragent.org/");
String userAgent = driver.findElement(By.cssSelector(".user-agent")).getText();

System.out.println(userAgent);
driver.quit();

These frameworks make it easy to get started with headless browser automation using just a few lines of code!

Headless Browser Use Cases

Here are some examples of real-world uses cases where headless browsers provide major value:

Web Scraping and Content Aggregation

  • Aggregating product data from ecommerce sites into search engines and comparison sites.
  • Compiling news articles and content from media sites into curated feeds.
  • Scraping dynamic data from JavaScript-heavy sites like social networks and blogs.

Test Automation

  • Cross-browser testing across Firefox, Chrome, Safari headlessly in parallel.
  • Stability testing – driving heavy load against web apps by simulating concurrent users.
  • Functional testing – automated form interaction, navigation, validation etc.

Rendering and Conversion

  • Converting designer mockups into static HTML for handoff using headless rendering.
  • Capturing rendered screenshots of responsive sites at multiple viewports for archives.
  • Generating PDF versions of pages, documents, and articles for download.

Web Archiving

  • Crawling expired sites to create browser-rendered archives for the Internet Archive.
  • Archiving important pages when companies or governments intentionally delete content.
  • Capturing dynamic content like comments that wouldn‘t be preserved statically.

Search Engine Crawling

  • Large search engines rely on highly optimized custom headless crawlers to index the web.
  • Google, Bing, Yandex, and Baidu all use hidden browser engines without UIs.
  • Allows efficiently rendering JavaScript pages and extracting link graphs.

Accessibility Testing

  • Headless browser testing can validate if pages conform to accessibility standards.
  • Checks can include validating ARIA roles, focus order, color contrast, etc.
  • Headless mode eliminates inconsistencies caused by browsers‘ native accessibility features.

Advanced Headless Browser Techniques

While the basics are straightforward, there are some advanced techniques that can optimize headless browser usage:

Distributed Testing

Headless browser testing can be distributed across many machines using a framework like Selenium Grid. This allows executing large test suites in parallel to reduce time to completion.

API Testing

Along with front-end UI testing, headless browsers can exercise back-end API endpoints through scripted interactions. Useful for integration testing.

Benchmarking

Running performance benchmarks headlessly eliminates inconsistencies caused by the browser UI. Provides clean, repeatable results across runs.

Stealth Mode

Using a proxy service like BrightData with headless browsers enables bypassing bot mitigation protections for successful scraping.

Container Deployment

Docker containers provide lightweight virtual environments to isolate and run headless browser instances. Streamlines configuration.

Cloud Testing

Frameworks like LambdaTest enable running massively scaled headless browser testing in the cloud across thousands of parallel nodes.

Conclusion

Headless browsers provide a versatile tool for programmatically accessing the modern web. They strike an excellent balance between automation capabilities and low resource usage.

For testing and scripting scenarios where a full browser UI is unnecessary overhead, headless browsers excel. Their ability to execute real standards-compliant web pages enables uses like testing, scraping, rendering, and archiving.

With the right techniques, headless browsers can be applied to solve nearly any problem requiring scripted access to the modern web platform. Their versatility and low resource demands make them an essential part of any web developer or tester‘s toolkit in the 2020s.

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