* Omit XML declaration:
* XML version is 1.0
* encoding is UTF-8
* standalone is no
so it can be omitted.
* Remove unnecessary attributes:
* `xmlns:rdf`, `xmlns:cc`, `xmlns:dc`, `xmlns:xlink` are not used in
these SVG icons.
* No text inside SVG, so `xml:space` is unnecessary.
* `version` attribute will be removed in SVG 2. [Changes from SVG 1.1 —
SVG 2](https://svgwg.org/svg2-draft/changes.html#structure)
* `x` and `y` can be omitted if they are `0`.
* Remove `enable-background` attribute / style property:
* Firefox / Safari does not support `enable-background`.
* It is proposed to be deprecated in SVG 2. [Filter Effects Module Level
1](https://drafts.fxtf.org/filters/#AccessBackgroundImage)
* Remove empty `g` elements.
* Add `width` and `height` attributes in `safari-icon16-off.svg`:
* They are in safari-icon16.svg, simple missing?
