Hello,
One client has offered me work - making actualization of his site.
The problem for me is that the site is totally in Javascript which I don’t know at all.
So I have offered him changing site to (X)HTML + CSS but I’m not sure if this is what client really want (or actually) need.
So my questions are:
What do you think about Javascript based sites?
What are the benefits of non-javascrpit site?
Maybe I shouldn’t take this job and leave it to sb else?
ralph.m and felgall - many thanks 
Great explanations.
I’ll sent you PM, if you both don’t mind.
Personally, I would say just tell the client what you can offer, and let him decide if that’s what he wants.
JavaScript is a good servant but a bad master. It should enhance a site but not take it over. That is, the content should be clear and accessible even with JavaScript (and CSS) turned off. If the site falls apart with JS turned off, it is a rubbish site. So the first thing I would suggest is test the current site with JS turned off, and if it falls apart, you can tell the client that you can make a better site that everyone can access.
Then, if he wants some JS enhancements, you can enhance the content with some jQuery tricks… although make sure that the content is still accessible with jQuery turned off.
What are the benefits of non-javascrpit site?
Well, it will probably load a bit faster; but really, there is no necessary advantage, except that a site with accessible content is much better than one whose content can only be accessed properly with JS turned on. Remember than not everyone has JS turned on, and some devices don’t support it at all.
Maybe I shouldn’t take this job and leave it to sb else?
If the client insists on having a lot of JS that is inaccessible or over your head, I would let him go. I recently turned down a lucrative offer because the client insisted on hopeless inaccessible JS enhancements. The site is now built, and I tested it with JS off, and it was a total disaster.
There’s no such thing as a JavaScript based site. Web sites must at the very least have HTML and all JavaScript can do is to interact with the HTML to make the site easier (or harder) to use.
HTML = content
CSS = appearance
JavaScript = behaviour
Of course if they are trying to use JavaScript to do something other than behaviour then their site is broken anyway since running or not running JavaScript is something that they have no control of.