Every web designer has already created several forms in his career. Probably a more or less complicated JavaScript code was included to check whether the entered values are valid and whether all required fields were filled in before sending the form.
It's a little easier now. With HTML5, input fields in Web forms can be compared with a pattern that describes the validity of the input when the browser enters them. For client-side form validation, you simply use the new attributes pattern and required. Together with CSS3 selectors for the optical effects, writing check mechanisms and reporting incorrect entries is greatly simplified. The patterns are like templates into which the entered data must fit.
Creating a pattern is a small hurdle for the website designer. Even if he uses a CMS, a blog software, an editing system, he (still) has to create the patterns himself. If finished solutions are already installed, he must at least check and understand them. According to the rules of World Wide Web Consortium (W3C), the Regular Expressions of ECMAScript Language (JavaScript) are used. But there are small differences, which are of course addressed in the tutorial.
With form patterns and a good browser you can test an astonishing amount, but still another check of the entered data, like for example the calculation of a check digit and the plausibility must be carried out additionally.
Of course it's the same as always. Not all browsers have implanted the check completely and correctly. But that will certainly improve in the future.