Among the development intelligentsia its bad reputation comes from the fact that it is a for-profit resource that is maintained by a closed group of people. We developers love open source.

Second, W3Schools is always woefully out of date. It always has been, and this is a consequence of it being a closed resource. There is a LOT of working constantly happening in the worlds of HTML, CSS, and JavaScript, and if a resource doesn’t keep up, it is a detriment to the programming world, not a benefit.

Third, W3Schools is almost always lacking in robustness of documentation. They even admit this on the bottom of the page in tiny text.

Examples might be simplified to improve reading and basic understanding.

Their descriptions of methods will often be completely missing descriptions of optional arguments, and their description of HTML and CSS often is missing huge considerations that are important for complex layouts. Again, this causes active harm.

To see the difference between a good resource and W3Schools, just compare pages on it to Mozilla Developer Network.