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Purchased as a teaching material for students of WEB design online school.I think that even beginners can understand and acquire the skills by reading this and working hard.Also, old-fashioned web developers like me can learn about recent trends that they have missed, so it's worth reading.
A validation service that I haven't used unexpectedly. Google Chrome extension (HTML error checkerEtc.), but sometimes you can try it.
HTML validation service
CSS validation service
So, when I check this page for a trial, an error is found violently.