Opera has updated the alpha version of its web browser with the release of Opera Developer 21.0.1432.5 for Windows and Mac. The new build fixes a few regressions, plus implements new tab-related tweaks to improve start-up times.

Version 21.0 opens with a number of important fixes, restoring Adblock to full working order and promising an end to the “Harlem Shake” when hovering over tabs on the Mac platform. It also fixes a number of unspecified crashes.

The big focus in this latest developer build is on improving start-up times where users are restoring a large number of tabs. The features are all accessible via the opera://flags shortcut – users should type the name of each feature (minus its hashtag) into the Experiments search box to switch it on.

Remember that Opera Developer is an alpha build, so use with caution.

The most popular new feature is #delay-onload, which prevents onload JS events from launching in background tabs until they’re selected. Opera’s developers have reported improved loading times and CPU/memory usage as a result of enabling this feature.

A new #extended-lazy-session-loading feature, which also requires #lazy-session-loading to be enabled, slows down overall loading times of sites when restored, but again speeds up startup and reduces the load on the computer’s CPU.

Another interesting feature is #tab-hibernation, which when enabled will suspend background tabs that exhibit no loading or streaming behaviour for 40 seconds, freeing up memory usage in the process. Users of low-bandwidth networks should also explore #session-tabs-serialization, a feature that will serialize pages to hard drive when the browser is closed, restoring them from there rather than the network.

Other recent additions to Opera Developer 21 include integration of Aura on Windows, which uses hardware acceleration to render pages and the browser user interface. Users also gain a setting to ensure the complete URL of a web page is displayed in the Address field (choose Settings > Advanced > Show always full URL in address field).

Mac users also gained full keyboard support for web apps, but users should note one known issue in this build is that no thumbnails are created when adding new speed dials.

Opera Developer 21.0.1432.5 is available now for testing for experienced Windows and Mac users. Also available are Opera 20 FINAL and Opera Next 20.0.1387.59. Both can be installed alongside the Developer build.