All modern web browsers support functionality to search for content on the active page. Most map the functionality to Ctrl-F, some also to F3 or just the dot-icon on the keyboard.
Google Chrome marks all matches on the page when you start to type automatically; the active match is highlighted in a different color (orange), the other hits on the page in yellow so that it is easier to find the current location on the page.
That's important, as you may use the shortcut or up and down search control buttons to jump to the next or the previous entry on the page.
Google plans to make search more useful by automatically searching for any highlighted text on a page when a search is launched.
Just double-click on any word or hold down the left mouse button to highlight text on a page, and press Ctrl-F or any of the other shortcuts to highlight any instance of the selected word or phrase on the page. Google searches for the text selection automatically when its search functionality is invoked.
The new option may speed up searches on a page provided that the search phrase can be highlighted on the page; this may be better than having to copy and paste the word or having to type it manually in the search box.
Google Chrome Canary supports the search improvement already, other Chrome versions will get it in the near future. Since the change is made to Chromium, other Chromium-based browsers will get it as well provided that they don't change it or remove it.
Vivaldi, a Chromium-based browser with lots of usability tweaks and features, supports the feature already.
Firefox does not support the feature to the best of my knowledge. Mozilla's web browser supports a highlight all feature just like all the other browsers though and Mozilla improved page search in 2016.
Browser extensions may extend the local search functionality of web browsers. Firefox users can check out Search Multi Tabs for that, the awesome FindBar Tweaks is unfortunately not compatible with Firefox 57 or newer, and that is also true for the extensions Hugo or Tabby.
Now You: how often do you use the integrated search tool of your browser?