"Rendr" which can rewrite CSS and HTML in real time and preview
If you click on the "CSS" tab and rewrite the code in it, it re-renders the entire page in real time without reloading. By clicking the "Markup" tab, you can rewrite the HTML code itself. Click "clear markup and css" to restore.
Firefox extension "GreaseMonkey"Version, so you can do something similar at your site, making trial and error of CSS design easier.
Details are as below.
The greaseMonkey version is designed to rewrite the HTML file of the local cache directly and perform realtime preview when the cache is valid.
To install, please install "greaseMonkey version" on the above page with greaseMonkey installed.
To use it, press the "Pause" key or "Break" key while displaying the page you want to rewrite. Then the window opens, just rewrite the code inside. It is very easy to try and error about the CSS of your page.