I wish to build the site on Wordpress like the ones, https://password-checker.io or https://passwordmonster.com. Where should I get started? Or simply I build in html/javascript to make the site which generates strong passwords and check password’s strength
Not sure why you’d need WordPress for what amounts to a single page website.
You’ll probably want to invest more of your time into figuring out how you’re going to generate those strong passwords.
I wonder why you would want to create such a site at all. Most browsers nowadays have a strong-password generator built-in, which automatically kicks in when a password must be created on a site.
Also, the urgent advice from all security advisors is to use a password manager (PM), to be able to have a different password for each site/program. Such PM apps all have a strong-password generator built-in as well.