Como posso alterar um texto de tag A específico ou aumentar o tamanho de uma tag DIV?
Existem extensões de navegador que podem lidar com isso com facilidade.
Os mais comuns são GreaseMonkey (somente Firefox) e TamperMonkey (que funciona em vários navegadores).
No seu caso (Safari 10.1), você precisa usar o TamperMonkey, pois ele funciona com o Safari > = 5:
Tampermonkey is a free browser extension and the most popular userscript manager. It's available for Chrome, Microsoft Edge, Safari, Opera Next, and Firefox.
Even though some of the supported browsers have native userscript support, Tampermonkey will give you much more convenience in managing your userscripts. It provides features like easy script installation, automatic update checks, a simple overview what scripts are running at a tab, a built-in editor and there is a good chance that incompatible scripts run fine with Tampermonkey.
Tampermonkey is available for Google Chrome, Microsoft Edge, Opera, Chromium, a lot of their derivatives like CoolNovo and Rockmelt and also Firefox and some Android browsers. It's installed in just a minute, so give it a try!
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Fonte TamperMonkey