In a Web-to-Lead form, what happens if required data is not provided?
For example, if you are creating a lead from Salesforce, these are the default required fields:
If the Last Name and Company value are not populated at the Web-to-Lead form, the user won’t receive any error during the submission. Instead, Salesforce will assign the ‘[not provided]’ value for Last Name and Company. For the other required Custom Field, the record will be created with empty value, there will not be any validation error during the record creation.