are you asking why it is bad as in "in what sense is it bad?" or as in "how did it end up being so bad"?
It's bad because it's missing most of the information it should have. look at https://www.w3schools.com/css/css3_flexbox.asp for instance, it has none of the reference materials it should have about flex, yet it is in the top 5 google results for "css flex". it's completely useless as a reference (or for learning, really). compare to https://developer.mozilla.org/en-US/docs/Learn_web_developme... which actually has the information on it, albeit in an annoying tutorial format.
It's bad because it's missing most of the information it should have. look at https://www.w3schools.com/css/css3_flexbox.asp for instance, it has none of the reference materials it should have about flex, yet it is in the top 5 google results for "css flex". it's completely useless as a reference (or for learning, really). compare to https://developer.mozilla.org/en-US/docs/Learn_web_developme... which actually has the information on it, albeit in an annoying tutorial format.