Mana Web — ruling 1

When a land an opponent controls is tapped for mana, this card checks which types of mana (white, blue, black, red, green, or colorless) that land could produce using any of its mana abilities, not merely the one the player just used. Then it checks which lands that player controls have abilities that could produce any of those types of mana. Then it taps those lands.

Mana Web — ruling 2

The lands that are tapped by Mana Web’s ability don’t produce mana as a result.

Mana Web — ruling 3

The opponent can tap their lands for mana in response to Mana Web’s ability triggering.

Mana Web : règles, fonctionnement et interactions (MTG)

Mana Web is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Whenever a land an opponent controls is tapped for mana, tap all lands that player controls that could produce any type of mana that land could produce.

How it works : Mana Web

Original English text — Wizards of the Coast / Scryfall

#1April 1, 2008· Wizards of the Coast
When a land an opponent controls is tapped for mana, this card checks which types of mana (white, blue, black, red, green, or colorless) that land could produce using any of its mana abilities, not merely the one the player just used. Then it checks which lands that player controls have abilities that could produce any of those types of mana. Then it taps those lands.
No other card is named in this ruling.
#2April 1, 2008· Wizards of the Coast
The lands that are tapped by Mana Web’s ability don’t produce mana as a result.
No other card is named in this ruling.
#3April 1, 2008· Wizards of the Coast
The opponent can tap their lands for mana in response to Mana Web’s ability triggering.
No other card is named in this ruling.