Go to the Safari Internet Search Box in the top menu bar. The one that says Google/Yahoo!/Bing
In that search field, type the word you're looking for on the page you're currently looking at. For example, on this page, you could search for your name, lordofthereef. DON'T press the search button on the keyboard. you will see a suggested search term, and under it, it shouf say "On This Page". You can click that.
This is how you search on *any* webpage. If you're searching for common words, such as "dog", you will have to scroll to the very bottom of the suggested search terms in order to reveal the "On This Page" section.