# SVG DOM preprocessing
Unlike the usual XML DOM implementations, `svgdom` will preprocess the document/tree a lot.
## XML parsing
`svgdom` uses [`roxmltree`](https://github.com/RazrFalcon/roxmltree) as an XML parser.
You can find its parsing details [here](https://github.com/RazrFalcon/roxmltree/blob/master/docs/parsing.md).
## Non-SVG elements and attributes
Only SVG 1.1 elements and attributes will be parsed.
But it's possible to write custom elements and attributes.
## `style` attributes splitting
From:
```xml
<svg style="fill:black;stroke:green;"/>
```
to:
```xml
<svg fill="black" stroke="green"/>
```
## Text unescaping
All [character references](https://www.w3.org/TR/xml/#NT-CharRef) will be resolved.
Not only simple one like `&` but also any hexadecimal and decimal.
Also, all whitespaces will be replaced with the Space character. Even escaped one.
The text data from the SVG like this:
```xml
<svg>
<text>  &@@&  <text>
</svg>
```
will be represented as `'@@'` and saved as:
```xml
<svg>
<text>&@@&</text>
</svg>
```
## Whitespaces trimming
The text data from the SVG like this:
```xml
<svg>
<text>
Text
</text>
</svg>
```
will be represented just as `Text` and not as `␣␣␣␣␣␣␣␣␣Text␣␣␣␣␣`.
`svgdom` also supports `xml:space` attribute. So the text data from the SVG like this:
```xml
<svg>
<text>
Text
<tspan xml:space="preserve"> Text </tspan>
Text
</text>
</svg>
```
will be represented as `Text␣␣␣Text␣␣Text`. And saved as:
```xml
<svg>
<text>Text <tspan xml:space="preserve"> Text </tspan>Text</text>
</svg>
```
Note that nested `xml:space` is mostly an undefined behavior and every XML DOM implementation
will process it differently. `svgdom` follows the Chrome behavior.
But Firefox, for example, will process the SVG above as `Text␣␣␣Text␣␣␣Text`.
## CSS resolving
`svgdom` supports only a tiny fraction of the CSS 2.1 features.
If the SVG contains an unsupported CSS, it will lead to a parsing error unless
the `ParseOptions::skip_invalid_css` is set.
After the preprocessing the `style` elements will removed.
From:
```xml
<svg>
<style type="text/css">
.fil1 { fill:green }
</style>
<rect class="fil1"/>
</svg>
```
to:
```xml
<svg>
<rect fill="green"/>
</svg>
```
The proper style resolving order is supported too.
From:
```xml
<svg>
<style type="text/css">
.fil1 { fill:blue }
</style>
<rect fill="red" style="fill:green" class="fil1"/>
</svg>
```
to:
```xml
<svg>
<rect fill="green"/>
</svg>
```
## Paint fallback resolving
SVG allows specifying the paint fallback value in case of an invalid FuncIRI.
For example:
```xml
<svg>
<rect fill="url(#gradient1) green"/>
</svg>
```
`svgdom` will convert it into:
```xml
<svg>
<rect fill="green"/>
</svg>
```
because there are no elements with a `gradient1` ID.
But more complex cases, like a reference to an invalid element, should be resolved manually.
For example:
```xml
<svg>
<linearGradient id="gradient1"/>
<rect fill="url(#gradient1) green"/>
</svg>
```
This will be represented as is, even though that `linearGradient` is invalid
(because have no children).
## Crosslink resolving
If an element is linked to itself, directly or indirectly, it may lead to a recursion/endless loop.
`svgdom` will resolve some simple cases by default.
In the example below the `lg1` gradient is linked to itself indirectly
via the `xlink:href` attribute.
From:
```xml
<svg>
<linearGradient id="lg1" xlink:href="#lg2"/>
<linearGradient id="lg2" xlink:href="#lg1"/>
</svg>
```
to:
```xml
<svg>
<linearGradient id="lg1" xlink:href="#lg2"/>
<linearGradient id="lg2"/>
</svg>
```
More complex cases should be resolved manually. Like:
```xml
<svg>
<pattern id="patt1">
<rect fill="url(#patt1)"/>
</pattern>
</svg>
```
## Namespaces
There is only one namespace: http://www.w3.org/2000/svg .
All namespaces will be resolved and removed.
So SVG like this:
```xml
<svg:svg svg:x='0' xmlns:svg='http://www.w3.org/2000/svg'/>
```
will became:
```xml
<svg xmlns='http://www.w3.org/2000/svg' x='0'/>
```
`xmlns` and `xmlns:xlink` attributes/namespaces will be added automatically.
http://www.w3.org/1999/xlink is a special case and will be handled too.