Spending UTXO from same block

Question

Block 546 is the first time a (actually two) transaction spends the UTXO of a transaction from the same block.

Is the order important? (The transaction that generates the UTXO has to appear before in the block than the transaction that spends it)

Is it still possible today?

Answer 1

The order must be causal. Most blocks today have outputs that were created earlier in the block.