Encryption with Web Crypto API (SubtleCrypto)
Is it a good idea [from a security perspective] to use the first 16 bytes of SHA-1 (20 bytes long) as IV?

It depends.
For CBC, the main requirements is the IV are that it should be unpredictable for the attacker. In particular, you should never ever use the same IV more than once for encrypting messages under the same key.
This implies that, in your system, it should be impossible to edit or overwrite these items. If editing an item is ever possible, then deriving the IV from the item identifier is definitely a bad idea.
If you want to support the editing of items, you should always generate a (pseudo-random) IV. (I.e. it should not be predictable for an attacker.)