Why I'm not using Joomla... and instead Bootstrap

Why not?

I've been using Joomla for 10 days.  I can see that it is a powerful CMS with more functionality that Wordpress, and it got some extensions that can reduce the work of coding.

However, I indicated in my brief that the purpose of my project was:

"Improve the quality of a user’s experience when members of Freegle groups are interacting online with ‘Freegle Direct’ (www.direct.ilovefreegle.org), to make it more intuitive and efficient.

1.   Redesign the site to improve its usability and to work across different size of screens, browsers and operating systems.

2.   Give the user more control to manage their account (profile, privacy, other accounts in social media and email preferences)

3.   Improve the form use to post Offer/Wanted items, including all the information required by users looking for items.

4.   Provide the right tools for users to manage their posts (Offer/Wanted/Events).

5.   Improve the layout to browse items."

My important point is to redesign the website in terms of using experience. If I try to invest my time in learn php, xlm and sql, as well as Joomla internal organisation variables, objects, modules, I'm not going to be able to test my product.
My priority now is to built a web prototype of my website using html and css, with dreamweaver.

Bootstrap Responsive design framework

http://mobile.smashingmagazine.com/2014/02/19/responsive-design-frameworks-just-because-you-can-should-you/

This article present the advantages of using a Design Framework as Bootstrap.  This need to be seen as a productivity tool, where "Think of it as a productivity tool that you can draw on in full or in part. Download just the grid system, or take the whole package with all of the extras. Perhaps use a framework just for prototyping purpose or actually reuse the parts of the framework that you know well in your own project.". It is also a powerful tool for prototype.

http://getbootstrap.com/getting-started/

Here I can get all the information

I'll customise first the framework before downloading: http://getbootstrap.com/customize/

I couldn't do it, I'll download the full version:

General info:

Grid system http://getbootstrap.com/css/

Grid systems are used for creating page layouts through a series of rows and columns that house your content. Here's how the Bootstrap grid system works:

• Rows must be placed within a .container (fixed-width) or .container-fluid (full-width) for proper alignment and padding.
• Use rows to create horizontal groups of columns.
• Content should be placed within columns, and only columns may be immediate children of rows.
• Predefined grid classes like .row and .col-xs-4 are available for quickly making grid layouts. Less mixins can also be used for more semantic layouts.
• Columns create gutters (gaps between column content) via padding. That padding is offset in rows for the first and last column via negative margin on .rows.
• Grid columns are created by specifying the number of twelve available columns you wish to span. For example, three equal columns would use three .col-xs-4.