Overall, this was an excellent article, but I'd like to point out a couple things.
First, a class like title seems a bit pointless when header tags should be used for that purpose. Two options would be using h1 as post-title and h2 as card-title in a parent/child sort of relationship OR using h2.post and h2.card if both are children of the same parent. Of course, if you're truly adventurous and find an edge case where header tags are inappropriate for your titles, you may just create 3 classes: title (containing styles that are the same for all titles), post (for styles unique to the post titles), and card (for styles unique to the card titles). Then all you have to do is assign multiple classes to the elements.
Second, over the years I've found plenty of buggy code, bad advice, and broken semantics on W3Schools. I've also found that anything I may have once looked for in W3Schools can be found in MDN, and that MDN gives accurate, updated information, marks experimental features and depreciated features accordingly, gives references to the W3C's standards, and includes a chart showing which browsers do and do not support the code shown on that page. If it were me, I'd just boot W3Schools out of the list of resources.
Aside from those two little things, this article is nothing but good advice ... for any language, really. I've got nothin' but ♡ for it.
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