When using stylesheets, go with external. The most significant improvement between internal and external CSS is download speed.
When a visitor requests your web page, the browser will look at the linked CSS file and download it. If you use the same CSS file on all your pages, then the browser will only download the CSS file once and used the cached version for subsequent pages.
For internal CSS, the CSS code will be downloaded everytime a page is requested by the browser, since it is part of the page and not separate.
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