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HTML 

              
                # SusyCSS: *Susy without Sass*

This is a near-complete implementation of 
[Susy 3](https://github.com/oddbird/susy/releases/tag/3.0.0.alpha.1), 
built entirely with CSS Variables (custom properties). 

[Check out the Demo »](https://codepen.io/mirisuzanne/pen/PboVrw?editors=0100)

(this pen is intentionally unstyled,
containing only CSS variables,
so that it can be pasted anywhere)

---

## Global Settings

These variables should be set
on the `:root` element,
so that they inherit.

- `--susy-columns: 4;`
  » [integer] 
  *available columns on the grid*
- `--susy-gutters: 0.25;`
  » [number] 
  *relative size of a gutter to a column*
- `--susy-spread: 0;`
  » [-1 | 0 | 1] 
  *number of span gutters in relation to columns*
- `--susy-container-spread: 0;`
  » [-1 | 0 | 1] 
  *number of container gutters in relation to columns*
- `--susy-debug-color: hsla(0, 0%, 0%, 0.125);`
  » [color] 
  *used to show columns when debugging*

For fully-static grids, set `--susy-static-grid`.
For static-gutters on an otherwise fluid grid,
set `--susy-static-gutters`.
If either value is true,
make sure you set the desired column-width.

- `--susy-static-grid: 0;`
  » [boolean] 
  *0 for fluid (%) output, 1 for static (based on column-width)*
- `--susy-static-gutters: var(--susy-static-grid);`
  » [boolean] 
  *0 for fluid (%) gutters, 1 for static (based on column-width)*
- `--susy-column-width: 5em;`
  » [length] 
  *width of one column, when using static output*


## Returned Values

These variables represent Susy's output —
like the return value from a Sass function.
They can be applied to any property,
or used inside `calc()` 
for more complex math.
Don't set these, just use them!

- `--span-width` (`--fluid-span` / `--static-span`)
  » *default, static, or fluid width for current span*
- `--gutter-width` (`--fluid-gutters` / `--static-gutters`)
  » *default, static or fluid width for current gutters*
- `--container`
  » *static width of the current columns context*


## "Mixins"

These variables work like mixins,
automatically applying returned values
to specific properties.

- `--span: initial;`
  » [integer] 
  *used to calculate the local value of `span-width`,
  and automatically applied to the `width` property.*
- `--gutters-inside: 0;`
  » [number]
  *any number of gutter-widths to apply as left/right padding*
- `--gutters-left: 0;`
  » [number]
  *any number of gutter-widths to apply as left margin*
- `--gutters-right: 0;`
  » [number]
  *any number of gutter-widths to apply as right margin*


## Debug Values

These values need to be used together,
applied to the `background` and `background-size`
properties respectively.

- `--debug-image` / `--debug-image-split`
  » *apply to `background` or `background-image` property
    to generate a gradient representing one column/gutter pair —
    use `-split` if you want half a column on each side.*
- `--debug-image-size`
  » *apply to `background-size` property
    in order to scale the background-gradient down,
    so it repeats properly across the grid.*

As always, 
the background grid gradient for debugging 
doesn't handle sub-pixel rounding. 
That's nothing new.
Maybe we could build an inline-svg background
using custom properties as well?
Something to try!

---

## Advantages

1. SusyCSS understands the DOM structure! *Settings actually inherit*!
2. No Sass required!


## Limitations

1. CSS has no necessary control structures, 
   so the syntax & settings are bulkier. 
2. SusyCSS doesn't support asymmetrical grids.
3. Within the context of a single selector,
   each variable can only have a single value.
   There is no way to pass different arguments.
4. Why build this much flexibility into a single code base?


## ToDo:

1. Can we make an inline-svg grid image?
!

CSS 

              
                :root {
  /* "--susy-" variables establish the grid */
  /* settings will inherit... */
  --susy-columns: 4;
  --susy-gutters: 0.25;
  --susy-spread: 0;
  --susy-container-spread: 0;
  --susy-debug-color: hsla(0, 0%, 0%, 0.125);
  
  /* for static grids only */
  --susy-static-grid: 0; /* boolean */
  --susy-static-gutters: var(--susy-static-grid); /* boolean */
  --susy-column-width: 5em;
}

* {
  /* ...but all the math has to happen on the element. */
  
  /* INTERNAL MATH */
  /* ------------- */
  
  /* container math */
  ---su-container-sum: calc( 
      var(--susy-columns) + 
      ( 
        ( var(--susy-columns) + var(--susy-container-spread) ) 
        * var(--susy-gutters) 
      )
    );

  
  /* span math */
  ---su-span-sum:  calc( 
    var(--span) + 
    (
      ( var(--span) + var(--susy-spread) ) 
      * var(--susy-gutters)
    )
  );
  
  
  /* gutter output math */
  ---su-inside-gutters: calc( 
      var(--gutter-width) * var(--gutters-inside) 
    );
  
  ---su-gutters-left: calc( 
      var(--gutter-width) * var(--gutters-left) 
    );
  
  ---su-gutters-right: calc( 
      var(--gutter-width) * var(--gutters-right) 
    );

  
  /* background grid math */
  ---su-grid-image-sum: calc( 
      1 + var(--susy-gutters) 
    );
  ---su-grid-gutter-ratio: calc( 
      var(--susy-gutters) / var(---su-grid-image-sum) * 100%
    );
  ---su-split-gutter-ratio: calc(
      var(---su-grid-gutter-ratio) / 2
    );
  
  
  /* PUBLIC FUNCTIONS */
  /* ---------------- */  
  
  /* span functions */
  --fluid-span: calc(
      var(---su-span-sum) / var(---su-container-sum) * 100%
    );
  --static-span: calc(
      var(---su-span-sum) * var(--susy-column-width)
    );

  --span-width: calc(
    var(--static-span) * var(--susy-static-grid) +
    var(--fluid-span) * (1 - var(--susy-static-grid))
  );

  
  /* container functions */
  --container-width: calc(
      var(---su-container-sum) * var(--susy-column-width)
    );
  
  
  /* gutter functions */
  --fluid-gutters: calc(
      var(--susy-gutters) / var(---su-container-sum) * 100%
    );
  --static-gutters: calc(
      var(--susy-gutters) * var(--susy-column-width)
    );
  
  --gutter-width: calc(
      var(--static-gutters) * var(--susy-static-gutters) +
      var(--fluid-gutters) * (1 - var(--susy-static-gutters))
    );
  
  
  /* debug functions */
  --debug-image: linear-gradient(to right, 
    var(--susy-debug-color, #ccc) calc(100% - var(---su-grid-gutter-ratio)), 
    transparent calc(100% - var(---su-grid-gutter-ratio)));
  --debug-image-split: linear-gradient(to right, 
    transparent  var(---su-split-gutter-ratio), 
    var(--susy-debug-color, #ccc) var(---su-split-gutter-ratio), 
    var(--susy-debug-color, #ccc) calc(100% - var(---su-split-gutter-ratio)), 
    transparent calc(100% - var(---su-split-gutter-ratio)));
  --debug-image-size: calc(
      var(---su-grid-image-sum) / var(---su-container-sum) * 100%
    );
  
  
  
  /* PUBLIC MIXINS */
  /* ------------- */  

  --span: initial; /* integer */
  --gutters-inside: 0; /* multiplier */
  --gutters-left: 0; /* multiplier */
  --gutters-right: 0; /* multiplier */

  
  /* output */
  width: var(--span-width, initial);
  padding-left: var(---su-inside-gutters, initial);
  padding-right: var(---su-inside-gutters, initial);
  margin-left: var(---su-gutters-left, initial);
  margin-right: var(---su-gutters-right, initial);
}
!

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