1.e4 c5 2.Nc3 Nc6 3.f4 g6 4.Nf3 Bg7 5.Bb5 (D)
This type of Closed Sicilian with an early f2-f4 by white is typically known as the Grand Prix attack. Sometimes white chooses a setup with g2-g3 and Bf1-g2, but with 5.Bb5 Webster signals his intention to give up the bishop pair to permanently damage black's pawn structure.
5. ... a6!?
It is widely considered here that 5. ... Nd4! is the strongest response for black. The point is simple: we avoid any Bxc6 ideas and threaten to take the bishop anyway "for free". After the most common continuation 6.0-0 (White has also tried 6.Nxd4!? cxd4 but this gives black a strong cramping pawn in the center) 6. ... Nxb5 7.Nxb5 black simply has the bishop pair and it's not immediately obvious where the compensation is. Typically white doesn't accomplish much in those positions.
With his last move black daringly declares his opponent's idea to be so worthless that he's even willing to waste a tempo to force him to execute it! If not 5. ... Nd4, I would have at the very least preferred 5. ... d6, which contributes to development and influences the center.
6.Bxc6 bxc6 7.0-0 (D)
7. ... d5?
Although it looks natural to expand into the center like this, in reality this hasty advance is a serious mistake. The issue is that if white ever starts attacking c5, there's no way black can support it with a pawn anymore - he will either have to resort to painfully passive contortions to hold on to it or just accept its loss. For this reason the restrained 7. ... d6 was correct.
A viable alternative was 8.e5! to close down the Bg7. This also would avoid a small escape that black could have taken on the next move.
8. ... e6
Black will be worse off no matter what, but I think the best shot is to try the endgame after 8. ... Bxc3 9.bxc3 dxe4 10.dxe4 Qxd1 11.Rxd1 Nf6 - the opposite colored bishops offer some drawing chances and at least white's structure is damaged too.
9.Qe1 Ne7 10.Qf2 (D)
It should be clear now that black is struggling to defend c5.
10. ... d4
Unfortunately there's not much else to suggest. 10. ... Qa5 runs into the unpleasant 11.Bd2, creating the threat of Nc3-xd5. Although we're only ten moves into the game, white is already completely dominating, all because of the weak c5 square. It is instructive to see the rest of the execution:
11.Na4 c4 Desperation. If 11. ... Qa5, then 12.b3 and 13.Ba3 will win the pawn anyway. 12.dxc4 Qa5 13.b3
Unfortunately there's no way black can take advantage of the exposed long diagonal. 13. ... d3 can even be met by the flashy 14.cxd3! Bxa1 15.Bd2 and 16.Rxa1 with dominating compensation for the exchange.
13. ... 0-0 14.Ba3 Re8 15.Rad1 Bd7 16.Bc5 Even though there's no pawn there anymore, it's still all about the c5 square! 16. ... Nc8 17.Nxd4 Bf8 18.Nf3 Qc7 19.Ne5 Rd8 20.Bxf8 Kxf8 21.Qh4 Kg8 22.c5 Rb8 23.Qf6 1-0
This is a good example of how just one or two hasty decisions in the opening can have a disastrous effect on the rest of the game. After 7. ... d5, white played almost flawlessly to realize his advantage.