If you need to look up a specific tag, attribute, rule or function of these languages, chances are you’ll find it in their reference sections, like this one here for the iframe tag. These pages provide definitions on the thing itself as well as all its attributes or arguments and also contain information about how it’s supported in popular browsers.
Their explanations always feature a little online code editor you can use to try out the effect of the commands you’re learning about.
W3Schools also offers tutorials you can use to learn these languages from scratch or dive deeper into a certain topic. It’s still a good idea to consider other resources for learning web development.
A few years ago, the website came under fire for using outdated examples and not explaining the actual reasons things worked a certain way. They’ve since improved, but some still argue that W3schools should not be your only source of learning material.
A good alternative is the Mozilla Developer Network, or MDN for short. It’s explanations go deeper into the workings of the programming language and can help you understand their rules a little better.
But for quick reference, W3Schools is much more extensive and descriptive, so it’s still a valuable resource, especially for beginners.