The webpage contain the following:
Treasure of the Pirates!It should in some of this (or hidden) island!
Hint: you need a map of this place to find it!
```Island #0Island #1<SNIP>Island #9998Island #9999```
We are looking for the map. We check robots.txt: it contains: Sitemap: /treasuremap.xml
The file /treasuremap.xml contain the location of the "treasure":
```<urlset><url><loc>7BmqJfhWhEa30NeVj7j2.html</loc><lastmod>2005-01-01</lastmod><changefreq>monthly</changefreq><priority>0.8</priority></url>```</urlset>
And the "treasure" contain the flag: flag{tr3asur3_hunt1ng_fUn}