A training section feature for beginners would be great. For ex., in teaching opening theory, there should be a intro for when each opening is to be used and then a computer generated exercise in the end where the user responds to the correct opening.
I know of websites with 'best move' or other techniques(pinning, skewing, discovery,etc.) exercises, but that's about it.
YouTube is good only if you know or heard of a opening and plus, there is no interactive exercise where a user can practice learning the openings.
If you do know of a website as such, please let me know, and disregard this topic.
There is not much in the way of teaching openings because masters generally advise a balance of studying chess in favor of learning the middle-game and endgame phases and tactics, rather than the opening phase.
I probably use 365chess.com the most, but it requires registration (and, if you want nag-free experience, payment), and the problems are maintained by douche-bags that just copy the analysis results off of Rybka or whatever and criticize +/- 1 Ply for mate in overly extended win sequences in master games.
A good alternative that's free is the Shredder materials resource:
Chess Puzzle of the Day (plus weekly puzzles)
Free Master Games Database
Standard Nalimov Endgame Tablebase:
Awesome, the shredderchess links are good. Kind of what I was looking for.
A website to interactively explore openings? I don't quite know of any other than what Mentos mentioned, but... I have borrowed a book from my university called "Catastrophe in the Opening" by Iakov Neishtadt
(the name sounds familiar, but I don't remember where... I think... 11. Ng4 sacrifice from that Berlin Karpov Gambit was proven unsound by him, and he's the one that suggested the move i think? off-topic stuffzzzzz).
Your best bet is definitely on YouTube, maybe even watch those 5 minute blitz from Channels like ChessNetwork, ChessExplained, and Kingscrusher to get a quick and overloading amount of (semi-useless) information on openings.
An alternative is to grab a chess board and run through things yourself. Use Chessgames.com and run through/analyze a bit of master games to grab ideas.
I personally haven't studied openings thoroughly yet, but all those videos on YouTube definitely gave a noticeably positive impression on my gameplay. See if you can find that book (library or online)! It'll go through what not to do in openings :P
Yeah the thing about openings is that, to humans, they're generally all a matter of personality, so you're told to think about which opening best meets your aggressiveness or other "chess personality" traits.
So it is a little bit about bias. After all, even the best opening moves according to computers are those that only a few grandmasters or top-level champions have agreed to adopt. The opening phase is otherwise entirely disputed amongst experts if not the opposite of that: agreed to be an insignificant matter of taste.
I generally rely more on mathematical or electronic intelligence after deciding what's interesting personally by myself to verify with that later. It always helps if you want to look into a specific opening that fascinates you, as hoping to "master the opening phase" is a bit too generic to pursue successfully.