Browser bookmarklets are small pieces of code that are stored as bookmarks. They enhance browsers that don't allow extensions, such as most mobile browsers. Installing them is relatively simple.
Video tutorial for iOS 11 (Safari).
This shows the HTML
text, this feature is called "Inspect Element" or "View Source" on desktop browsers.
This changes all the text on the current page to 9px. Combined with Font Plus [src]
to increase the size and Font Minus [src]
to decrease it makes this even more powerful.