Page caching speeds up the first visit to your site. Browser caching speeds up every visit after that. Together, they are the one-two punch of web performance — and both should be enabled on every WordPress site.
What Is Browser Caching?
When a browser loads a web page, it downloads the HTML, CSS, JavaScript, images, fonts, and other assets that make up the page. Without browser caching, the browser downloads all of these assets again on the next visit — even if nothing has changed. With browser caching, the server tells the browser “you can keep this file for a week” or “this file will not change for a month,” and the browser loads it from its local cache instead of downloading it again.
The result: repeat visitors see your pages load almost instantly, because the heavy assets — images, stylesheets, scripts — are already on their device.
Cache-Control Headers Explained
Browser caching is controlled by HTTP headers — specifically, the Cache-Control header. The server includes this header with every response, telling the browser how long it can keep the file. A typical configuration might set CSS and JavaScript files to cache for one year, images for one month, and HTML to not cache at all (so content updates are visible immediately).
You do not need to understand the headers to benefit from them. Caching Out adds the correct Cache-Control and Expires headers with a single toggle. The defaults behind that toggle are sensible for every WordPress site, and you can adjust the durations if you have specific requirements.
Browser Caching + Page Caching = Maximum Speed
Page caching makes the first visit fast. Browser caching makes the second visit faster. Together, they mean that a visitor who comes to your site, reads a blog post, and clicks through to another page experiences near-instant loads on every page after the first. The HTML is served from the page cache, and the CSS, JavaScript, and images are served from the browser cache. The server does almost no work.
Further reading: How Page Caching Works (and Why Your Site Needs It) for the server-side half of the caching equation.