There are two interpretations of your question, which do you mean?<p>1. What is wrong with leaving HTML completely unstyled and allowing the browser to apply its defaults?<p>2. Why is it that the default settings applied by (most) browsers are so awful?<p>The answer to the first is pretty much, as another HNer noted, the topic of the Better Motherfucking Website page. The defaults suck.<p><i>Why</i> the defaults suck is ... probably an accident of history that&#x27;s difficult to undo. Browsers default to no margins and a whole slew of other stuff, and <i>CSS requires that you either start from known defaults or explicitly style each and every element.</i><p>If every browser used its own distinct set of deviations-from-default for various entities, CSS would be <i>even more</i> of a hack than it already is.<p>Some of the fault also surely lies with standards organisations, including the W3C. If <i>they</i> elected for specific defaults to be applied to all pages, and, by some miracle, browser vendors actually adhered to these, you could conceivably have a world in which <i>most</i> Web pages <i>didn&#x27;t need</i> much if any styling, and where users could choose to apply default styling to pages.<p>I do this presently via uBlock and uMatrix on desktop, and via ReaderMode on Mobile. I virtually always prefer the simplified, standardised view to a site&#x27;s native design.<p>You&#x27;ve asked a simple question, but a deep one.