Answer.

Cross-browser testing is the automation of tests to control the display and functionality of a website across different browsers and their versions.

Background:

In the early days of web applications, websites were often tested manually across all major browsers, where developers could not ensure a consistent display of elements. Later, tools (Selenium Grid, SauceLabs, BrowserStack) emerged that allowed for the automation of repetitive checks across different browsers and platforms using a single set of tests.

Problem:

Differences in the implementation of HTML/CSS/JS standards among browsers
Constantly changing browser versions and updates
The need for quick execution of a large number of tests across multiple configurations

Solution:

Using Selenium Grid or cloud providers (BrowserStack, SauceLabs) for parallel execution of automated tests in various browsers and versions
Setting up a testing platform that supports the most demanded combinations (browser and version selection strategy)
Integrating with CI/CD pipelines for automatic runs after each release/change

Key features:

Automated execution of the same scenario across multiple browsers
Parallelism — speeding up execution through scaling
Monitoring of real user configurations and quick adaptation of the test matrix

Tricky Questions.

Can you completely abandon manual testing if there are cross-browser automated tests?

No. Automated tests cannot cover rare or subjective UI bugs (pixel-perfect layouts, non-standard fonts), some issues are only identified manually.

Is it enough to just run tests on all versions of browsers?

No. It is necessary to analyze the target audience; based on real user statistics, select a limited number of supported versions, otherwise, the testing cost will uncontrollably increase.

Should cross-browser checks be integrated with the main automated testing system?

Yes. If cross-browser checks are not built into the overall pipeline, there is a high likelihood of forgetting to run them or not noticing bugs on certain browsers.

Common Mistakes and Anti-Patterns

Lack of a strategy for selecting browser versions
Manual execution of this type of tests
Ignoring real user statistics
Insufficient parallelism

Real-Life Example

Negative Case

Cross-browser tests are run manually "on holidays" only in Chrome and in the latest version, cloud services are not used. It turns out that after the next release, the site displays incorrectly in Safari only for some users.

Pros:

Fast, minimal infrastructure
Low load on CI

Cons:

Bugs slip into production
Real user browsers are not taken into account
High cost of fixing post-factum

Positive Case

An automated test run in BrowserStack is used based on a pre-selected matrix of browsers (Chrome, Firefox, Safari, Edge — the last 2 versions of each), tests are integrated into CI/CD, and results are automatically analyzed.