Card counting in online blackjack is a strategy used by players to gain an advantage over the house by keeping track of the ratio of high to low cards remaining in the deck. Unlike traditional casinos, online platforms often use multiple decks and automatic shuffling, making card counting more challenging. However, skilled players can still apply techniques like the Hi-Lo system to estimate the likelihood of favorable hands. While not illegal, many online casinos employ measures to detect and discourage card counting, which can lead to account restrictions or bans for those caught using this strategy.