For all modern browsers a separate CSS file with background images with the help of data:URI is created. This file can be called on deferred window.onload and loaded with other images (or even after them). The second CSS file is created especially for IE5/6/7, and it's called via conditional comments. This file contains all background images encoded with mhtml. With these 2 approaches with can deliver all images in base64 format for 99.9% browsers (except IE7 for Vista and some old ones).
It analyzes HTML code of entered URL for website (or separate webpage), finds all CSS files (and embedded styles within style), finds all background images from these files and merges CSS code into one (optional). Then all rules related to background images are removed from the main file and this file is optimized with YUI Compressor (optional). Background images are encoded with base64 and added to 2 files: for all modern browsers (addon.css) and for IE (addon.css.js). Then all additional files are inserted before the end of HTML file, and into the head call to optimized main CSS file replaces the original one(s). Then an archive with all required files is formed and ready to download.
File addon.css contains all rules for modern browsers (Firefox, Opera, Safari, Chrome, IE8) about background images and images themselves encoded with base64.
File addon.ie.css.js is included for IE only (with conditional comments) in deferred(defer="defer") mode. It means that this file is loaded only after DOM tree for the whole document is ready (more information can be found in the article about deferred load mode for all browsers). This allows you to show web page for current users as soon as possible. JavaScript file inserts all required rules to the document's head on its load (and they are used JavaScript file itself as the source for background images). This allows you to minimize number of requests to the server to load all design-related data.
File addon.std.css is called in IE if user has JavaScript disabled (so all background images are called as is). Also this file is included for VIsta IE7.
With an option "Compress CSS" all source CSS files will be optimized with YUI Compressor. This allows you to decrease overall size of CSS files up to 20–30%.
"Add Gzip" option added to the archive with results .gz versions of final CSS files. This can be used with static archives (with Apache or nginx you can use files .gz to serve compressed versions of your files).
"MD5 Name" option adds for every CSS file name unique MD5 hash that is matches all used background images. This allows you not to replace existing files that have not been changed.
What does option "For Vista IE7 document.onready / document.write" mean?
At this moment we know the only issue with DURIS technology — it's Vista IE7 and non-supported mhtml. To resolve this problem we target this combination of OS / browser with JavaScript (and file for disabled user with JavaScript contains calls to original background images' files). document.onready option allows you to inject additional file on DOM ready event and document.write will include CSS file call directly to the preferred place in HTML file (footer or header). The last choice can be useless for websites that use AHAH.
What does option "Limit size of images 4k / no limits" mean?
Browsers have some limits for data:URI / mhtml maximum file size. Also large amount of base64 encoded data included into CSS file will increase its size. So usually background images included into CSS file are now greater than 4 Kb. If you are sure about disabling such restriction you can choose no limits option.
DURIS adds calls of all required CSS files directly to HTML code. TO speed up website load you can separate CSS files with background images' data from CSS rules and "delay" their load. This can be accomplished with deferred window.onload and dynamic CSS files injecting. More examples of DURIS usage.