Using the template-driven approach, form validation is a matter of following HTML5 practices:
<!-- a required field -->
<input type="text" ... required>
<!-- an optional field of a specific length -->
<input type="text" ... pattern=".{3,8}">
<!-- a non-optional field of specific length -->
<input type="text" ... pattern=".{3,8}" required>
<!-- alphanumeric field of specific length -->
<input type="text" pattern="[A-Za-z0-9]{0,5}">Note that the pattern attribute is a less-powerful version of JavaScript's RegEx syntax.
There are other HTML5 attributes which can be learned and applied to various types of input; however in most cases they act as upper and lower limits, preventing extra information from being added or removed.
<!-- a field which will accept no more than 5 characters -->
<input type="text" maxlength="5">You can use one or both of these methods when writing a template-driven form. Focus on the user experience: in some cases, it makes sense to prevent accidental entry, and in others it makes sense to allow unrestricted entry but provide something like a counter to show limitations.