npm package: @marp-team/marpit-svg-polyfill
Description: Polyfill for inline SVG slides rendered by Marpit
Author: Marp team
Latest version: 2.1.0

# @marp-team/marpit-svg-polyfill

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The polyfill for [the inline SVG slide][inline-svg] rendered by [Marpit].

[marpit]: https://github.com/marp-team/marpit
[inline-svg]: https://marpit.marp.app/inline-svg

### Supported browser

- [WebKit](#webkit) based browser: Safari and iOS browsers (including iOS Chrome, iOS Firefox, iOS Edge, and so on...)

## Usage

```html
<!-- Generated HTML by Marpit's inline SVG mode -->
<div class="marpit">
  <svg viewBox="0 0 1280 720" data-marpit-svg="">
    <foreignObject width="1280" height="720">
      <section>...</section>
    </foreignObject>
  </svg>
</div>

<!-- After than, use polyfill through jsDelivr CDN -->
<script src="https://cdn.jsdelivr.net/npm/@marp-team/marpit-svg-polyfill/lib/polyfill.browser.js"></script>
```

## Why need?

### WebKit

[Marpit]'s [inline SVG slide][inline-svg] has a lot of advantages: No requires JavaScript, gives better performance for scaling, and has predicatable DOM structure.

But unfortunately, WebKit browser has not scaled the wrapped HTML correctly. It is caused from a long standing [bug 23113](https://bugs.webkit.org/show_bug.cgi?id=23113), and it does not resolved in the last 15 years.

![](https://raw.githubusercontent.com/marp-team/marpit-svg-polyfill/main/docs/webkit-bug.png)

> **Note**
> A brand-new SVG engine for WebKit called as [**"Layer-based SVG engine (LBSE)"**](https://blogs.igalia.com/nzimmermann/posts/2021-10-29-layer-based-svg-engine/) is currently under development, and it will finally bring glitch-free scaling without JS. (See also: [Status of the new SVG engine in WebKit](https://wpewebkit.org/blog/05-new-svg-engine.html))
>
> You can test LBSE in [Safari Technology Preview](https://developer.apple.com/safari/technology-preview/) by following these steps:
>
> 1. Install Safari Technology Preview
> 1. Run `defaults write com.apple.SafariTechnologyPreview IncludeInternalDebugMenu 1` in terminal
> 1. Open Safari Technology Preview
> 1. Turn on **"Layer-based SVG engine (LBSE)"** from "Debug" menu → "WebKit Internal Features"
> 1. Restart app
>
> marpit-svg-polyfill v2.1.0 and later will try to detect whether or not enabled LBSE, and does not apply polyfill if LBSE was available.

## Solutions

### For Webkit

#### Scaling

We try to simulate scaling and centering by applying `transform` / `transform-origin` style to Marpit `<section>` elements.

```html
<svg viewBox="0 0 1280 960">
  <foreignObject width="1280" height="960">
    <section style="transform-origin:0 0;transform:matrix(......);">
      ...
    </section>
  </foreignObject>
</svg>
```

marpit-svg-polyfill uses the result of `getScreenCTM()` method, so the polyfill will sacrifice "zero-JS slide", the key feature of inline SVG.

#### Repainting

WebKit browser would not trigger repainting even if modified the contents of slide. It becomes a problem when supporting the live preview feature in Marp tools.

Fortunately, [a genius already resolved this problem only in CSS!](https://stackoverflow.com/a/21947628) `transform:translateZ(0)` would trigger re-painting immidiately when modified contents.

#### Animation GIF

People like to put GIF animation in the slide. However, GIF in polyfilled slides have glitches. GIF updates only a cropped part somewhere.

Applying `transform:translateZ(0.0001px)` to each `<section>` elements within SVG is a magic to resolve that. 🪄

> **Warning**
> This style brings slightly blurred contents too. Our polyfill prefers to render animated contents correctly.

<!--

## Advanced

### Apply polyfill manually

You may pick out the logic of polyfill if you required. When called a picked function, it applies polyfill forcibly without browser detection.

```javascript
import { webkit } from '@marp-team/marpit-svg-polyfill'

const observer = () => {
  // Apply polyfill for webkit forcibly
  webkit()

  window.requestAnimationFrame(observer)
}

document.addEventListener('DOMContentLoaded', observer)
```

#### Use case in Blink browsers

We have confirmed a similar rendering bug to WebKit in a few Blink based browsers. (e.g. Chrome 66, Electron 3.x. refs: [marp-team/marpit#35](https://github.com/marp-team/marpit/pull/35), [marp-team/marp-cli#15](https://github.com/marp-team/marp-cli/pull/15))

We are not applied polyfill for Blink browsers because [they are working toward to resolve this.](https://bugs.chromium.org/p/chromium/issues/detail?id=467484) But you may apply `webkit()` manually if you required.

-->

## Contributing

We are following [the contributing guideline of marp-team projects](https://github.com/marp-team/.github/blob/master/CONTRIBUTING.md). Please read these guidelines this before starting work in this repository.

## Author

Managed by [@marp-team](https://github.com/marp-team).

- <img src="https://github.com/yhatt.png" width="16" height="16"/> Yuki Hattori ([@yhatt](https://github.com/yhatt))

## License

This module releases under the [MIT License](LICENSE).