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How do I add a blur BEHIND an html element?

I've dug through the internet and found some things that seem to be going in the right direction but nothing that seemed like a complete solution. Here's what I found:
The upcoming CSS property backdrop-filter, which is currently only supported in Safari and behind a Chrome developer flag. Exactly what I need, but it needs better support.
The JavaScript library Blur.js, which seems to have all but disappeared from the internet. (Even the official website leads to a GoDaddy page.) It may have been a solution but I'm not sure what happened to the project.
The JavaScript library StackBlur, which seems to be a brilliant blurring solution, but I haven't the slightest idea how I would use it to blur BEHIND an element.
Any ideas? Perhaps someone has used StackBlur to do this and has a CodePen I could poke through or something? Basically, I have a grid of element each using jQuery UI Draggable that I would like to make translucent with a nice blur behind each one. The blur needs to update as expected when the elements are dragged around.
One last thought. Right now what I'm using is a pure CSS "solution", but it has some annoying limitations. If you are going to suggest a solution with only CSS it's probably what I'm already doing. The issues with it are the following:
The area behind the element isn't really blurred, just an arbitrary background image.
The edges or the blurred area do not cleanly cut off the blur. This is a bit hard to describe, but just trust me when I say that it's doesn't look great.
Unless I missed something huge in all my searching, I think I'm going to need JS for this one.

Barmar is correct. Give two objects the same absolute position. Blur one object and assign it a lower z-index. Bob's your uncle.
If you need some clearer specifics, then please post your code following the SO guidelines.

battle tested jquery / javascript drop down menus

Drop Down menus are a common and almost ubiquitous UI element, and yet, I find it really hard to "pick one". before heading down the long road of rolling and "battle testing" my own, I'd like to put it out there and find out what others are using.
What are the most popular solutions for jquery, or javascript dropdown menus ?
Should be, lightweight, easy to customise and properly cross browser compatible, and at least function appropriately down to IE8.
Ones I've tried and used in the past with the best experience that meet this criteria inlcude :
Super fish - http://users.tpg.com.au/j_birch/plugins/superfish/
Dynamic Drive Smooth Menu http://www.dynamicdrive.com/dynamicindex1/ddsmoothmenu.htm
To Clarify: looking specifically for multilevel drop down menus similar to the examples above. I'm also open to "avoid these they're sh*t" type answers if you feel so inclined.
PS: please avoid adding 5 from one of the 500 drop down menu articles that appear when first searching on google, I'd like to know about what YOU"RE using, what dropdown menus YOU love and hate.

have you tried this may be this can give you what you want
http://www.erichynds.com/jquery/jquery-ui-multiselect-widget/

I personally have used SuperFish, in it's standard form, it's not too pretty but with quite a bit of adjustments I find it can look quite nice.
Then using something like this: http://snook.ca/archives/javascript/jquery-bg-image-animations
Demo: http://snook.ca/technical/jquery-bg/
Can make them look very cool!

After a fair amount of research and messing about, quite honestly I'm fairly disappointed with the options in this area there are a couple options but as commented above rolling my own seems to be the best way forward.

Spinning wheel in jquery with mouse-drag option

I am looking for a plugin for spinning wheel in jquery. I came across the site
http://www.professorcloud.com/mainsite/carousel.htm . There is a beautiful demonstration of spinning wheel here but I need the images to be controlled on mouse-drag and also I want the same rotation to happen vertically as well. please help

I believe what you are looking for is commonly referred to as a 'Developer'. When you come to Stackoverflow (in general) you provide a source code that we look through to find out where your mistake(s) is/are. Then we tell you why the mistake(s) happen/ed, and steer you through correcting the mistake(s). If what you are you looking for is the type(s) of tool(s) that a common jQuery developer would use to achieve this then that's completely different.
For starters, it looks like you have found a carousel that you want to use to create the rotating effect. That's fine. You'll need to go through their API and instructions to get a grasp on how to integrate your next step:
Drag & Drop in jQuery
My favorite plugin for this is jQuery UI. It's a complete suite for jQuery that provides an array of different enhancements, including the 'Drag & Drop' effect you want.
http://jqueryui.com/demos/droppable/
You'll have to (once again) work with the API and instructions, and create events to control how the carousel should be affected by the Drag & Drop plugin.
Vertical Carousel
Some quick digging regarding the need for Vertical implementation lead me to a Stackoverflow question regarding the same exact needs.
JQuery Cloud Carousel that scrolls vertically
He also discussed what changes you need to make to the script in order to account for the change in axis. That's a huge +.
You can see a working version of it below -
http://www.inspiritandintruth.com/ (look at the right hand side)
All of this combined should get you what you want. With a bit of work and adjusting, you'll be able to get your end result.
Best of luck.

How to split page and reveal stuff underneath?

I've already achieved this on my iPhone app, but I want to know if it's possible on an HTML page, maybe using CSS effects or similar.
As you can see, the current view is split, the bottom part is moved down, and another view is revealed underneath. I have a page I'd like to try this on. Any ideas if this is possible, and any specifics as to how I can do it? I'm quite new to HTML coding, so please take it easy on me. :)
Thanks in advance!

Here's an example to get you started http://jsfiddle.net/Cquhj/
A few things to take away from this pattern:
The middle div has an overflow: hidden; property and height: 0px.
The trigger icon has an event that tweens the height of the middle div to the size you want.
Edit:
I really like the resources and answers given and I would add this to the list http://wiki.forum.nokia.com/index.php/Mobile_Design_Pattern:_Accordion_Menu

here an update, more iphone-like
http://jsfiddle.net/mFeyn/1/
it miss the triangle in the bottom of the folder once is clicked and calculate the height of the container when there is more than 4 icons.

Yes, it's absolutely possible, nothing out of the ordinary and CSS will definitely be needed.
As it is, your question is extremely generic and an answer would be: learn about HTML and CSS and the combination of the two for creating standard compliant web page layouts. You might want to read about the box model too. To solve your problem you need to know about the use, positioning and floating of a series of <div>s to achieve the desired layout.
If you want to add animation, like some part of the split view floating down into position, you will need Javascript as well.
Possible starting points for your research on SO:
Why not use tables for layout in HTML?
https://stackoverflow.com/search?q=css+div+column

Here is a code example that might give you a little bit more if your plan is to emulate iOS 4 folder behaviour using jQuery.
The view is basically split into rows and I played around w/ the background position css attribute to allow the background split illusion.
http://jsfiddle.net/hKHWL/

This is very possible, but it's kind of like asking "I want to program Civilization, and I'm quite new to C; how do I do it?" ;-)
I would strongly recommend picking up a good "DHTML" (Dynamic HTML) book. For instance, I rather enjoyed this one, from SitePoint: http://www.sitepoint.com/books/dhtml1/
If you're not the book-buying type, sites like SitePoint and AListApart can certainly explain things too, but not in as organized of a format.
Good luck.

I know this is an old post/question...
but I'm doing this with dynamic heights and positions here:
http://webkit-os.pixelass.com/iframe/
(only works in Chrome and Safari)
I am using jQuery and two divs with the same image.
Dynamic positions means.. you can move the folder to a different position or page.
Dynamic height means... the height is relative to the number of Icon-rows in the folder.
The folder even opens above and below if the content is too hight to be displayed below.
(opening the folder from the Dock does not work yet)

Rotating images + content - jQuery or MooTools?

I've found a feature on two different websites that I'd like to include on one of my web projects, but I can't figure out how they're doing it. I think they're using either jQuery or mootools, but I'm not sure.
http://www.x-plane.com/index_desktop.html
I'm new here so I don't have the reputation to post the other link, but if you do a Google search for "Andrews Institute of Orthopaedics" its the top result. I'll see if I can answer a few questions to earn my keep.
The thing that I like about these is that the images and text (including links) are all rotated together. I've done a few web searches related to cycle, rotate and slide but it's not coming together for me. Can anyone give me a nudge in the right direction?

The page you linked is using prototype for this (usually because that's what the author knew, as with most libraries), and it is custom script they've written. However, jQuery has a lot of plugins with various styles, just see what one fits your taste best. I'd suggest taking a look here and a look here.
Anything you find can be automated on a timer like the site you linked, or use buttons, or both. These are all pretty flexible, I'd start by finding the plugin you want, try and get it going, then asking a question here if you have problems getting it to behave exactly how you want.