Where is the index.html on OSX 10.9 (Mavericks) Apache setup?
There should be 3 files in /Library/WebServer/Documents:


index.html.en
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If these files are missing, it really shouldn't matter.

As soon as you create your own HTML file and place it in that folder, the "It works!" message should be replaced with your new index.html file.

Hope this helps.