# SVG Guidelines

## Pros and cons of SVG for images

When used as a document format there is usually a compelling reason that
makes SVG the only solution. When used as an [image
format](https://developer.mozilla.org/en-US/docs/Web/SVG/SVG_as_an_Image),
it is sometimes less obvious whether it would be best to use SVG or a
raster image format for any given image. The vector format SVG and
raster formats like PNG both have their place. When choosing whether or
not to use SVG it is best to understand the advantages and disadvantages
of both.

File size

: Whether SVG or a raster format will produce a smaller file for a
  given image depends very much on the image. For example, consider an
  image of a path with a gradient fill. The size of an SVG of this
  image will be the same regardless of the dimensions of the image. On
  the other hand the size of a raster file of the same image will
  likely vary tremendously depending on the dimensions of the image
  since the larger the dimensions the more pixel data the file needs to
  store. At very small dimensions (the extreme case being 1px x 1px)
  the raster file will likely be much smaller than the SVG file since
  it only needs to store one pixel of data. At large dimensions the
  raster file may be much larger than the SVG file.

Scalability, with caveats

: One of the primary advantages of SVG is that as it is scaled it does
  not pixelate. However, this is not to say that it always does away
  with the need to have a collection of raster images for display at
  different scales. This can be particularly true for icons. While SVG
  may scale well enough for flat-ish icons without a lot of detail, for
  icons that try to pack in a lot of detail graphic artists generally
  [want to be able to pixel
  tweak](https://www.pushing-pixels.org/2011/11/04/about-those-vector-icons.html).

Performance

: While SVG provides a lot of flexibility in terms of scaling,
  themability, etc. this flexibility depends on doing computations for
  SVG images at the time they're displayed, rather than at the time the
  author creates them. Consider an image that involves some complex
  gradients and filters. If saved as a raster image then the work to
  rasterize the gradients and filters takes place on the authors
  computer before the result is stored in the raster file. This work
  doesn't need to be redone when the image is displayed on someone
  else's computer. On the other hand, if the image is saved as an SVG
  image then all this work needs to be done each time the SVG is
  displayed on someone else's computer. This isn't to say that SVG
  images are always slower than raster equivalents. In fact it can be
  faster to send vector information from an SVG to a user's GPU than it
  is to extract raster data from an equivalent raster image. And even
  when an SVG image is slower than a raster equivalent, the difference
  is usually not noticeable. However, just don't fall into the trap of
  thinking that SVGs are faster than equivalent raster images, or vice
  versa. Once again, "it depends".

## Authoring guidelines

A lot of SVG files (particularly those generated by SVG editors) ship
without being cleaned up and can contain a ton of junk that bloats the
file size and slows down rendering. In general the best way to combat
this is to first run SVG files through a linter such as
[svgo](https://github.com/svg/svgo) (see the Tools section below).
However, when authoring SVGs by hand here are some best practices to
help keep them lightweight. These rules are based on some real examples
seen in Mozilla's code.

### Basics

- Two spaces indenting
- No useless whitespaces or line breaks (see below for more details)
- Adding a license header
- Use double quotes

### Whitespace and line breaks

#### Whitespace

In addition to trailing whitespace at the end of lines, there are a few
more cases more specific to SVGs:

- Trailing whitespaces in attribute values (usually seen in path
  definitions)
- Excessive whitespace in path or polygon points definition

#### Whitespace examples

This path:

```html
<path d=" M5,5    L1,1z ">
```

can be cut down to this:

```html
<path d="M5,5 L1,1z">
```

Similarly, this polygon:

```html
<polygon points="  0,0   4,4  4,0  "/>
```

can be cut down to this:

```html
<polygon points="0,0 4,4 4,0"/>
```

#### Line breaks

You should only use line breaks for logical separation or if they help
make the file readable. You should avoid line breaks between every
single element or within attribute values. It's recommended to put the
attributes on the same line as their tag names, if possible. You should
also put the shortest attributes first, so they are easier to spot.

### Unused tags and attributes

#### Editor metadata

Vector editors (Inkscape, Adobe Illustrator, Sketch) usually add a bunch
of metadata in SVG files while saving them. Metadata can mean many
things, including:

- The typical "Created with *editor*" comments
- Non-standard editor specific tags and attributes (`sketch:foo`,
  `illustrator:foo`, `sopodi:foo`, …)
- The [XML
  namespace](https://developer.mozilla.org/en-US/docs/Web/SVG/Namespaces_Crash_Course)
  definition that comes with the latter (`xmlns:sketch`,
  `xmlns:sopodi`, …)

#### Other metadata

In addition to non-standard editor metadata, standard compliant metadata
also exists. Typical examples of this are `<title>` and `<desc>`
tags. Although this kind of data is supported by the browser, it can
only be displayed when the SVG is opened in a new tab. Plus, in most of
the cases, the filename is quite descriptive So it's recommended to
remove that kind of metadata since it doesn't bring much value.

You shouldn't include DOCTYPEs in your SVGs either; they are a source of
many issues, and the SVG WG recommends not to include them. See [SVG
Authoring
guidelines](https://jwatt.org/svg/authoring/#doctype-declaration).

#### Avoid the use of CDATA sections

[CDATA
sections](https://developer.mozilla.org/en-US/docs/Web/API/CDATASection)
are used to avoid parsing some text as HTML. Most of time, CDATA isn't
needed, for example, the content in `<style>` tags doesn't need to be
wrapped in a CDATA section as the content inside the tag is already
correctly parsed as CSS.

#### Invisible shapes

There are two kinds of invisible shapes: The off-screen ones and the
uncolored ones.

The offscreen shapes are hard to spot, even with an automated tool, and
are usually context aware. Those kinds of shapes are visible but off the
[SVG view
box](https://developer.mozilla.org/en-US/docs/Web/SVG/Attribute/viewBox).
Here's [an
example](https://hg.mozilla.org/mozilla-central/diff/9fb143f3b36a/browser/themes/shared/heartbeat-star-lit.svg)
of a file with offscreen shapes.

On the other hand, the uncolored ones are easier to spot, since they
usually come with styles making them invisible. They must meet two
conditions: they must be devoid of any fill (or a transparent one) or
stroke.

#### Unused attributes on root `<svg>` element

The root `<svg>` element can also host many useless attributes. Here's
an
[example](https://hg.mozilla.org/mozilla-central/diff/2d38fecce226/browser/components/loop/content/shared/img/icons-10x10.svg)
taking into account the list below:

- `version`
- `x="0"` and `y="0"`
- `enable-background` (unsupported by Gecko and now deprecated by the
  Filter Effects specification)
- `id` (id on root element has no effect)
- `xmlns:xlink` attribute when there are no `xlink:href` attributes
  used throughout the file
- Other unused [XML
  Namespace](https://developer.mozilla.org/en-US/docs/Web/SVG/Namespaces_Crash_Course)
  definitions
- `xml:space` when there is no text used in the file

#### Other

- Empty tags, this may be obvious, but those are sometimes found in
  SVGs
- Unreferenced ids (usually on gradient stops, but also on shapes or
  paths)
- `clip-rule` attribute when the element *is not* a descendant of a
  `<clipPath>`
- `fill-rule` attribute when the element *is* a descendant of a
  `<clipPath>`
- Unreferenced/Unused clip paths, masks or defs
  ([example](https://hg.mozilla.org/mozilla-central/diff/2d38fecce226/toolkit/themes/shared/reader/RM-Plus-24x24.svg))

### Styling

#### Styling basics

- Privilege short lowercase hex for colors
- Don't use excessive precision for numeric values (usually comes from
  illustrator)
- Use descriptive IDs
- Avoid inline styles and use class names or SVG attributes

##### Styling examples

Here are some examples for excessive number precision:

- 5.000000e-02 → 0.05 (as seen
  [here](https://hg.mozilla.org/mozilla-central/diff/2d38fecce226/browser/themes/shared/devtools/images/tool-network.svg#l1.31))
- -3.728928e-10 → 0 (as seen
  [here](https://hg.mozilla.org/mozilla-central/diff/2d38fecce226/browser/themes/shared/aboutNetError_alert.svg#l1.12))
- translate(0.000000, -1.000000) → translate(0, -1) (as seen
  [here](https://hg.mozilla.org/mozilla-central/diff/2d38fecce226/browser/themes/shared/heartbeat-icon.svg#l1.13))

As for descriptive IDs:

- For gradients: SVG_ID1 → gradient1 (as seen
  [here](https://hg.mozilla.org/mozilla-central/diff/2d38fecce226/browser/themes/shared/aboutNetError_alert.svg#l1.12))

#### Use of class names

- Avoid using a class if that class is only used once in the file
- If that class only sets a fill or a stroke, it's better to set the
  fill/stroke directly on the actual shape, instead of introducing a
  class just for that shape. You can also use SVG grouping to avoid
  duplicating those attributes
- Avoid introducing variants of the same file (color/style variants),
  and use sprites instead (with class names)

#### Default style values

There's usually no need to set the default style value unless you're
overriding a style. Here are some commonly seen examples:

- `style="display: none;"` on `<defs>` elements (a `<defs>`
  element is hidden by default)
- `type="text/css"` on `<style>` elements
- `stroke: none` or `stroke-width: 0`

### SVG grouping

#### Style grouping

Group similarly styled shapes under one `<g>` tag; this avoids having
to set the same class/styles on many shapes.

#### Avoid excessive grouping

Editors can sometimes do excessive grouping while exporting SVGs. This
is due to the way editors work.

##### Nested groups

Avoid multiple-level nesting of groups, these make the SVG less
readable.

##### Nested transforms

Some editors use `<g>` tags to do nested transforms, which is usually
not needed. You can avoid this by doing basic algebra, for example:

```xml
<g transform="translate(-62, -310)"><shape transform="translate(60, 308)"/></g>
```

can be cut down to:

```xml
<shape transform="translate(-2,-2)"/>
```

because: -62+60 = -310+308 = -2

### Performance tips

These rules are optional, but they help speeding up the SVG.

- Avoid using a `<use>` tag when that `<use>` tag is being
  referenced only once in the whole file.
- Instead of using CSS/SVG
  [transforms](https://developer.mozilla.org/en-US/docs/Web/SVG/Attribute/transform),
  apply directly the transform on the path/shape definition.

### Tools

Tools can help to clean SVG files. Note, however that some of the rules
stated above can be hard to detect with automated tools since they
require too much context-awareness. To this date, there doesn't seem to
be a tool that handles all of the above. However, there are some
utilities that cover parts of this document:

- Mostly complete command line tool: <https://github.com/svg/svgo>

- Alternatives to SVGO:

  - <https://github.com/RazrFalcon/svgcleaner>
  - <https://github.com/scour-project/scour>

- GUI for command line tool (use with "Prettify code" and "Remove
  `<title>`" options on): <https://jakearchibald.github.io/svgomg/>

- Good alternative to SVGO/SVGOMG:
  <https://petercollingridge.appspot.com/svg-editor>

- Fixes the excessive number precision:
  <https://simon.html5.org/tools/js/svg-optimizer/>

- Converts inline styles to SVG
  attributes: <https://www.w3.org/wiki/SvgTidy>

- RaphaelJS has a couple of utilities that may be useful:
  [raphael.js](https://dmitrybaranovskiy.github.io/raphael/)