Make `html5ever` compilable on beta #4696

Job	Run time
Test codegen backend benchmarks	1m 25s
Test on Windows	2m 15s
Test benchmarks (--include cargo-0.87.1,stm32f4-0.15.1, Check,Debug,Doc)	6m 12s
Test profiling	3m 39s
Test benchmarks (--include cargo-0.87.1,stm32f4-0.15.1, Opt)	7m 3s
Compare page endpoint test	4m 47s
Test benchmarks (--exclude cargo-0.87.1,stm32f4-0.15.1, Check,Debug,Doc)	12m 43s
Test benchmarks (--exclude cargo-0.87.1,stm32f4-0.15.1, Opt)	11m 17s
Test and deploy	1m 24s
Database Check	1m 21s
Test runtime benchmarks	6m 13s
docker	2m 18s
conclusion	3s
	1h 0m 40s

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