Split testing CSS is the key.
A website’s main CSS file is a central point of reference which controls the common styles, positions and behaviours of all elements across each and every webpage.
Within the main CSS file are the settings for all fonts, margins, colours, alignments and so much more. It’s the most important file on most templated websites and can drastically change the look of a website with just a tiny edit.
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With thanks to Hallam for this article.