On having a closer look at the source code of a web page or the CSS file optimized by the Google Pagespeed apache/nginx extension, you'll find code of the following type "data:image/png;base64,iVBORw0KGgo". This string in HTML pages is called as Base64 data or data URI. Browsers convert this code into images.
Base64 reduces the DNS lookup times and the number of requests made to the server. This is because the images will be transferred inline as a part of the HTML page and not as a separate file
If you display an image via URL, the browser will request the web-server to transfer the file. If you use base64 encoding, no such request will be made to the server.
Although base64 encoding helps in improving the pagespeed, it two disadvantages. The first disadvantage is that the base64 encoding increases the size of the HTML pages. Even though you can enable GZIP compression to reduce the HTML size, you should use it only to inline small images that are being used repeatedly on your website. The 2nd disadvantage is that the images won't be indexed by Googlebot.
Enter the Base64 data in the text area displayed on this page and click the submit button. If the data URI is valid, Crawlcenter will display an image below the "Submit" button. To save the image on your computer/laptop, right-click on it and choose the "Save Image As" option.