The update brings mostly security and bug fixes and includes the changes from Android 4.4.4 as well.

htc one m8 update 4.4.3 HTC One M8 getting updated to Android 4.4.3

HTC has started updating its latest flagship to Android 4.4.3, with the update rolling out to unlocked and developer variants in the US.

The update comes in at a hefty 592 MB, but there is no mention in the changelog about any features except the bump to Android 4.4.3, which is strange given how large the over the air update is. Users should note that HTC will not be rolling out an update to Android 4.4.4, as the company has incorporated all fixes from 4.4.4 into the 4.4.3 update.

That means that the next major update to come knocking will be the Android L update, which HTC has committed to release for its flagship handsets within 90 days of the official release from Google. The Android 4.4.3 update is currently available over the air and should make its way to unlocked One M8’s around the world over the coming days, though anyone unwilling to wait can try and grab it manually from the phone’s Settings » About phone » Software updates menu.

Source: XDA