You'll need to create a Content Management System, or CMS, and pretty much reinvent the wheel that Wordpress, Joomla, and Blogger already are.
You'll need some sort of server side language to accept input (whether it be images, blog posts, etc) to save them on the server, PHP can do this, but it's still PHP. Javascript cannot do that by itself, neither can HTML or CSS as I'm sure you know.
I highly recommend against rewriting the wheel like this for a few reasons. Primarily security. You could easily make a few mistakes in the log in page, subtle mistakes that don't even seem like mistakes that somebody could abuse to gain entry to the site's content. What good is a gorgeous site if you can't lock it down? The site would quickly lose integrity.
Maintaining a site like this can also be a problem for a single person such as yourself. The simplest CMS is the most rigid, but I imagine she'd want control over rigidity. If you're doing this yourself, this becomes difficult quickly. This will take a lot of time and effort. Time and effort you won't be paid for.
If you do undertake this, she's going to have requests and you will quickly turn into a business relationship when it comes to the site. She'll be your client, always making requests asking you for help when it doesn't do what she expects it to. This is stressful/annoying in work environments, I can't imagine what it'd do to a relationship.
Can this be done using purely HTML/Javascript/CSS? No.
Can this be done with a little PHP? Yes.
Should it? I recommend against it. Find a Wordpress-esque site that gives you the control she wants and go with it. It'll save you a lot of time, effort, and maybe even the relationship, you never know. If you still want to design the look of her site, many of these sites allow for that but still have the powerful, secure, and maintained-by-other-people backend that will make the site that much easier to blog for.