Safari doesn't run javascript/css properly - not loading websites

Hi!
I'm so desperate now because my Safari stopped working properly a few weeks ago. I think there should be something wrong with the javascript and maybe with the css, but actually I don't know, I'm not that kind who knows the correct meaning of these IT words.

The first problem was with the Facebook's chatbox that the text-box got totally messed up. Now I experience a problem like this with every website which has interactive surface like the chatbox (eg. airbnb's site). Everything gets messed up and not displayed properly or the websites doesn't load fully, only the small loading picture appears on that part of the site where the interactive script should work. It happens only in my default browser Safari, everything works perfectly with Google Chrome and Opera. I've already tried nearly everything, I reset the browser, deleted history, empty the cache, banned the extensions. In the Development menu JS/CSS is not disabled, and in Preferences-Privacy Javascript is enabled too. I tried to delete and reinstall the whole browser too, but nothing helped.

I'm sorry for being so dumb, I know I'm not a genius at computer but I really hope somebody could help me. I want to get back my perfect Safari. Thank you for the answer!

@Vulusz, we need a lot more information. First what model is your Mac and what year, and what version is the operating system. If you don't know please follow these steps.

1. click on the Apple icon in the upper left hand corner. Select "About This Mac".
The operating system is listed at the top. Just below it is the version number. Note this in your reply.

2. in that popup window, click on the "System Report" button.
Note the "Model Identifier" and put this in your reply.

3. in Safari click on Safari, then select "About Safari"
Note the version number and send in your reply.

ManoaHi said:

@Vulusz, we need a lot more information. First what model is your Mac and what year, and what version is the operating system. If you don't know please follow these steps.

1. click on the Apple icon in the upper left hand corner. Select "About This Mac".
The operating system is listed at the top. Just below it is the version number. Note this in your reply.

2. in that popup window, click on the "System Report" button.
Note the "Model Identifier" and put this in your reply.

3. in Safari click on Safari, then select "About Safari"
Note the version number and send in your reply.

Click to expand...

Sorry that I missed out these informations, I have a MacBook Pro 13" Late 2011 model with OS X Mavericks 10.9.5 and Safari 9.1.3. I know I could upgrade to a better operating system, but I don't want to because of the new style and I'm not so sure about it would be useful for an old computer like mine.
I was thinking about if it would help to reinstall the Mavericks because I still have the original installer, but I haven't heard about reinstalling an operation system on Mac.

Once we get the above information, we're going to take you through Safari preferences.

In the Safari menu item, select "Preferences"
Then select the "Security" icon (the one that looks like a lock with black and yellow stripes.
Note whether or not "Enable JavaScript" is checked or not.

Next select the "Privacy" icon. Please indicate which option is selected in "Cookies and website data".

Please send that to us.

Javascript is enabled at the Security tab (the bracket is ticked). Under the Privacy page Cookies and website data is blocked from third parties and advertisers (sorry, my system is not english, the first line is checked).

Change last one to "Allow from websites I visit". That is the second to last one above "Manage Website Data..." button, or third choice overall. In the Develop menu have you played around with the Experimental Features? Do you have a specific URL that you're experiencing this with? I don't remember back in Mavericks and your MacBook Pro Late 2011 will run Sierra.