Pseudo-elements in CSS are used to apply styles only to a specific part of a web element, not to the complete element. They can be used to add styles such as colors, fonts, and effects to certain parts of an HTML element. Pseudo-elements are defined using a double colon (::) followed by the pseudo-element name. For example, `:first-line` or `::before`. The most commonly used pseudo-elements in CSS include `:first-letter`, `::before`, and `::after`. These pseudo-elements can be used to add styles to specific parts of an HTML element, such as the first letter of a word or the content before or after an element. Pseudo-elements are supported by all modern browsers but may not work in older versions of Internet Explorer. Selenium WebDriver can be used to automate tests on web pages that use pseudo-elements, but it requires the JavaScriptExecutor interface to interact with these elements. The JavaScriptExecutor can be used to retrieve the value of a pseudo-element using the `getComputedStyle` function and the `getPropertyValue` method. The cloud-based Selenium Grid like LambdaTest can also be used to run tests on multiple browsers and environments, including those that support pseudo-elements, ensuring cross-browser compatibility. By using the JavaScriptExecutor and the cloud-based Selenium Grid, developers can ensure that their web applications work correctly across different browsers and devices.