Auto merge of #13401 - notriddle:master, r=pcwalton

Implement sequential fallback to float speculation

This shouldn't impact any pages that are already rendering correctly, but it is a very naive implementation of this pass.

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #13284 and fix #13223
- [X] There are tests for these changes

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tests/wpt/metadata-css/css21_dev/html4/block-formatting-contexts-015.htm.ini

@@ -1,3 +0,0 @@
-[block-formatting-contexts-015.htm]
-  type: reftest
-  expected: FAIL

tests/wpt/metadata-css/css21_dev/html4/floats-bfc-002.htm.ini

@@ -1,3 +0,0 @@
-[floats-bfc-002.htm]
-  type: reftest
-  expected: FAIL