zh-cn: proofreading and update "Cross browser testing" section

[hanyujie2002]

I plan to proofread and update the translation of Multimedia and embedding section

Tasks

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1. zh-cn: revise translation of "Cross browser testing" #20008
   l10n-zh +1
2. zh-cn: revise the translation of "cross browser testing intro" #20137
   l10n-zh +1
3. zh-cn: revise the translation of testing stratege #20408
   l10n-zh +1
4. [zh-cn] update the translation of "Handling common HTML and CSS problems" #20811
   l10n-zh +1
5. Handling common JavaScript problems WIP
6. Handling common accessibility problems
7. Implementing feature detection
8. Introduction to automated testing
9. Setting up your own test automation environment