Fluid Grid

What is it?

Fluid, Responsive and Semantic grid is built for Sass (SCSS) and LESS CSS. It supports any number of columns. Gutter width is defined as percentage (rendered as left margin), and grids can be nested too. View demo

Requirements for a reusable, fast and flexible grid system

I think a CSS grid system should meet the following requirements so that it can be used as a reusable build tool in every project:

Flexible, supporting any number of columns and gutter widths, as required by the design (I don't want any default grid)
Allows fast grid prototyping (requires CSS preprocessor)
Fluid, percentage-based (fixed width grids can be easily generated by giving the containing row a fixed width, if needed)
Responsive, mobile first approach, applying the grid only if there's enough screen estate
Nested but keeping the same root grid classes (I want to able to split the container row by the number of columns the container has; it doesn't make sense to have class "grid-6" inside a parent that has a class "grid-6" if I want to split that parent in two. Instead, I want to use the same class than in the root level: "grid-3". This is different than what the Twitter Bootstrap or ZURB Foundation currently do).
Backwards compatible, cross-browser support (preferably incl. IE7+), no JavaScript dependancies
Right to Left language support easily added
Ability to decide whether to use unsemantic ".grid-x" classes, or my own.

So basically what we have here is The Semantic Grid (http://semantic.gs/) and Trevor Davis' Sass & Compass Grid (http://viget.com/inspire/building-a-nested-responsive-grid-with-sass-compass) merged together in Sass (SCSS), or alternatively with LESS CSS.

Grid preview tool

I've also created a grid configurator that you can use to try out both fluid and flexible grids, by entering the number of columns and gutter width (click on the plus sign in the top left corner, to expand the widget). To see your grid in a fixed viewport size, select one of the predefined device pixel dimensions from the dropdown.

Examples of usage

The grid is based on rows that can be nested inside other rows. Each row can contain multiple columns, up until the number of columns in the parent column.

For example, let's say you want 12 columns with 4% gutter (default settings). Put this in your cols.less stylesheet:

// First level 
.col {
    margin: 0 0 0 @gutter;
}
.two {
    .columns(2);
}
.three {
    .columns(3);
}
.four {
    .columns(4);
}

Alternatively, if you use Sass (put this in /scss/_grid.scss):

// First level
.col {
    margin: 0 0 0 $gutter;
}
.two {
    @include columns(2);
}
.three {
    @include columns(3);
}
.four {
    @include columns(4);
}

That compiles to:

// First level 
.col {
    margin: 0 0 0 4%;
}
.two {
    width: 13.333333333333334%;
}
.three {
    width: 22%;
}
.four {
    width: 30.666666666666668%;
}

And here's the markup:

<div class="row">
    <div class="col three"></div>
    <div class="col three"></div> 
    <div class="col four"></div>
    <div class="col two"></div>    
</div>

You can use your own semantic class names instead, if you fancy that. For example (LESS version):

// First level 
.navigation {
    .columns(2);
}	
.complementary, 
.contentinfo {
    .columns(3);
}
.main {
    .columns(4);
}

The same in SCSS:

// First level
.navigation {
    @include columns(2);
}
.complementary,
.contentinfo {
    @include columns(3);
}
.main {
    @include columns(4);
}

The markup:

<div class="row">
    <section class="col main"></section>
    <aside class="col complementary"></aside> 
    <nav class="col navigation"></nav>
    <footer class="col contentinfo"></footer>    
</div>

One more example: a nested grid, three levels deep. Your cols.less stylesheet:

// First level 
.col {
    margin: 0 0 0 @gutter;
}
.five {
    .columns(5);
}
.seven {
    .columns(7);
}

// Second level
// .nestedcolumns(children, parent);
.five .two {
    .nestedcolumns(2, 5);
}
.five .three {
    .nestedcolumns(3, 5);
}
.seven .three {
    .nestedcolumns(3, 7);
}
.seven .four {
    .nestedcolumns(4, 7);
}

// Third level 
.seven .four .two {
    .nestedcolumns(2, 4);
}

Same in SCSS:

// First level
.col {
    margin: 0 0 0 $gutter;
}
.five {
    @include columns(5);
}
.seven {
    @include columns(7);
}

// Second level
// nestedcolumns(children, parent);
.five .two {
    @include nestedcolumns(2, 5);
}
.five .three {
    @include nestedcolumns(3, 5);
}
.seven .three {
    @include nestedcolumns(3, 7);
}
.seven .four {
    @include nestedcolumns(4, 7);
}

// Third level
.seven .four .two {
    @include nestedcolumns(2, 4);
}

That compiles to:

// First level 
.col {
    margin: 0 0 0 4%;
}
.five {
    width: 39.333333333333336%;
}
.seven {
    width: 56.66666666666667%;
}

// Second level
// .nestedcolumns(children, parent);
.five .two {
    margin-left: 10.169491525423728%;
    width: 33.89830508474576%;
}
.five .three {
    margin-left: 10.169491525423728%;
    width: 55.932203389830505%;
}
.seven .three {
    margin-left: 7.0588235294117645%;
    width: 38.8235294117647%;
}
.seven .four {
    margin-left: 7.0588235294117645%;
    width: 54.11764705882353%;
}

// Third level 
.seven .four .two {
    margin-left: 13.043478260869565%;
    width: 43.47826086956522%;
}

The markup:

<div class="row">
    <div class="col seven">
        <div class="row">
            <div class="col four">
                <div class="row">
                    <div class="col two"></div>
                    <div class="col two"></div>
                </div>
            </div>
            <div class="col three"></div>
        </div>        
    </div>
    <div class="col five">
        <div class="row">
            <div class="col three"></div>
            <div class="col two"></div>
        </div>
    </div>    
</div>

In most cases, it's unlikely that you need more than three levels of grid nesting. (I haven't tested this grid deeper than that, but it should work.)

Generating a new grid

You can generate any number of columns just by changing two variables.

To generate a new grid in LESS CSS, do the following:

Uncomment and override the default values of @columns and @gutter variables in styles.less and styles-ie.less, to suit you needs.
Adapt your grid classes in /less/cols.less and call the .columns(@num) and .nestedcolumns(children, parent) mixins. The reason for having a separate cols.less file is that this way we can reuse the same column styles for oldIE (<IE9), including those styles without @media queries in styles-ie.less.
Make sure your markup has the correct classes.
Compile LESS files to CSS.

To generate a new grid in Sass (SCSS), do the following:

Change the values of $columns and $gutter variables in /scss/_grid.scss, to suit your needs.
Adapt your grid classes in /scss/_grid.scss and call the columns(@num) and nestedcolumns(children, parent) mixins.
Make sure your markup has the correct classes.
Compile SCSS files to CSS.

Credits

This grid system wouldn't exist without the many existing awesome grid frameworks. See the credit section in grid.less and _grid.scss.

I'm also using here Nicolas Gallagher's Mobile first CSS and getting Sass to help with legacy IE technique (http://nicolasgallagher.com/mobile-first-css-sass-and-ie/), to serve the same grid for oldIE (<IE9) that don't support @media queries. IE has min-width specified so the grid is fluid only until that point. More capable browsers get the responsive version.

There are many grids, but I use this one because it gives me the flexibility I need. Give it a try, to see if it can help streamlining your workflow too.