Null image source check for src, srcset and picture parent
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This PR includes a test which checks for empty src only.
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Add some support for WPT tests in an Android emulator through WebDriver
This succeeds on my machine:
`./mach test-wpt --product servodriver --binary etc/run_in_headless_android_emulator.py --binary-arg servo-x86 --binary-arg target/i686-linux-android/release/servo.apk /_mozilla/mozilla/DOMParser.html`
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```
0:27.48 pid:4157 Traceback (most recent call last):
0:27.48 pid:4157 File "/home/simon/servo1/etc/run_in_headless_android_emulator.py", line 212, in <module>
0:27.48 pid:4157 sys.exit(main(*sys.argv[1:]))
0:27.48 pid:4157 File "/home/simon/servo1/etc/run_in_headless_android_emulator.py", line 72, in main
0:27.48 pid:4157 forward_webdriver(adb, args)
0:27.48 pid:4157 File "/home/simon/servo1/etc/run_in_headless_android_emulator.py", line 163, in forward_webdriver
0:27.48 pid:4157 ports = [int(part) for part in split if part]
0:27.48 pid:4157 ValueError: invalid literal for int() with base 10: 'None'
```
```
Traceback (most recent call last):
File "/home/simon/servo1/tests/wpt/web-platform-tests/tools/wptrunner/wptrunner/testrunner.py", line 90, in run
rv = commands[command](*args)
File "/home/simon/servo1/tests/wpt/web-platform-tests/tools/wptrunner/wptrunner/testrunner.py", line 110, in wait
self.executor.wait()
File "/home/simon/servo1/tests/wpt/web-platform-tests/tools/wptrunner/wptrunner/executors/base.py", line 209, in wait
self.protocol.base.wait()
AttributeError: 'ServoWebDriverProtocol' object has no attribute 'base'
```
textarea minLength/maxLength
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textarea minLength/maxLength
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add TextLength
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add TextLength in HTMLTextAreaElement
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Respect the source rectangle when drawing a canvas to another canvas.
This allows us to run the three.js examples without panicking, since they use self-drawing canvases to scroll the FPS graph.
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improve spec compliance of window.close
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Improve the spec compliance of https://html.spec.whatwg.org/multipage/window-object.html#dom-window-close, mainly by implementing the steps related to [closing a browsing context](https://html.spec.whatwg.org/multipage/window-object.html#close-a-browsing-context)
---
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Fix for document salvageable state
I noticed a bug where if the document was un-salavageable due to the presence of `beforeunload` listeners, this could be overwritten if there were no listeners for `unload`. This could also have happened if an iframe of the document had no listeners while the parent did.
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