You can find a particular word or phrase on a web page in Safari on iPhone. With the web page open, tap the top URL/search bar and type in the desired phrase. You'll see a list of search results from the web, and other locations, but at the bottom of that list you'll see "On This Page", with the number of matches. Tap it, and you'll see that the results are highlighted in yellow. Tap the up-down arrows at the bottom of the screen to scroll through the instances.