According to the order of execution rules, the first step that happens when a record is saved is the JavaScript validation on the browser, if the record contains any dependent picklist fields. This validation limits each dependent picklist field to its available values. No other validation occurs on the client side. The other steps happen later on the server side, after the record is submitted to the server. References:
* Triggers and Order of Execution, Apex Developer Guide, Section: Triggers and Order of Execution
* Learn Salesforce Order of Execution, Salesforce Ben, Section: Order of Execution from Developer Docs API v54