I need your advice. I have a site and want to create a blog within it. I'd like to use WordPress as CMS, but I don't know PHP.
So, the question is: is it enough to know just JS and HTML for such purpose? Thanks in advance
Styphon's comment on your question was a pretty good summary. Adding my 2 cents worth!
You say "I have a site and want to create a blog within it" - while you can rely on WordPress for part of a website, I wouldn't recommend it - it involves making WordPress do things it wasn't intended to do and will get you closer to. Much safer to port your existing website content to pages within your WordPress installation.*
The way that the WordPress ecosystem currently looks, you could develop multiple WordPress sites and not touch a line of PHP, especially if you're willing to pay for pro editions of various plugins from time to time. The currently landscape has more than a few visual page builders, all in one themes and frameworks that sit on top of WordPress that do almost anything you want with a few clicks of the mouse.
However...the purist developer in me has to add this... If you want WordPress to work as quickly and cleanly as possible, minimising code bloat and overly complex engineering, it's worth learning a bit of PHP, at least enough to be comfortable theming WordPress from a boilerplate theme (and not an all-in-one theme).
* If you planned to do something with Angular or another JS framework, pulling data from the WordPress API, you can disregard this comment... But if you're comfortable enough with Angular, basic PHP will be easy!