what browser plugin that lets you override html of a website automatically
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Can anyone suggest a browser plugin that will let me override a website whenever I am loading a particular website.

**Example Scenario:**

Whenever I load yahoo.com I want to change on how it renders the html and behave.

 - It needs to be able to manipulate the DOM. example, it can change the attribute of an `<img height="" width="" />`, example the width and height of the image.
 - Aside from changing attributes it also need to be able to inject new elements. example i can insert new `<divs>` or `<p>`.
 - I also need to be able to manipulate the head tag as I like to insert external CSS and JS for the overrides.

I primarily use Google Chrome and Mozilla Firefox.

Thank You.