Amusingly, the top picture on their page mixes 2- and 4-space indentation:
<a href="https:&#x2F;&#x2F;18f.gsa.gov&#x2F;assets&#x2F;blog&#x2F;css-guide&#x2F;css-guide.png" rel="nofollow">https:&#x2F;&#x2F;18f.gsa.gov&#x2F;assets&#x2F;blog&#x2F;css-guide&#x2F;css-guide.png</a>