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Documentation
CSS selectors and XPaths are a very easy way to select mandatory elements on a website. When using CSS there are there standard selectors that are often used.
The CSS selector checker knows two content types it can be applied on. DOM and HTML. The DOM (document object model) is the representation of the rendered website. All JavaScript code was called, ads were loaded and lazy loading was triggered for the viewport.
The HTML document is the initial content that was created by the web server and was transferred to the browser. This content is more stable because no changes by the browser were done.