When it reaches form validation, there are a few things you should keep in mind:
1. Importance of the transmitted information;
2. How reliable it needs to be;
3. How many requests you are going to have;
4. How much information your form is going to hold;
In my honest opinion, I would use Javascript/Ajax for quick and more accurate paths in your form. For example:
1. Validating date fields are in correct format;
2. Validating ages are matching your requisitions;
3. Validating a gender was selected (BTW it is gender you are looking for, not sex);
etc…
Anything that needs to be correct and secure on your website (specially passwords and emails, etc), if you want to validate such you should do it on your server. Always!
Now, if you form is too long, you can do a step by step registration, instead of all in one. You can keep the progress saved in the session (server side) and maybe use a cookie to inform your site that they started the registration before (for this you can use javascript);
NOTE: You should inform your user you are using cookies as it is mandatory due to GDPR 🙂
Both validations help making the user journey smoother. There are things you should defo use as Js validation for quick response and making sure user is aware of some mistakes before they submit the form.
Server validation is safer for the sensible data that you need to make sure is correct as user as no say in it.
But it really depends on what you are trying to achieve and how many resources you have available to you.
Hope it helped clear some things up! 🙂
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solved Need advise. How to validate forms with AJAX and not to create excess load on server?