This question pertains to CSS and is not really relevant to android. The webkit on android is quite capable and should not cause any issues with your webviews.
In order to align something to the bottom of a container, the container (in this case, your li element) must have relative or absolute positioning and be a block-level item (either display: block; or inline-block with potential limitations). The span element then needs to have absolute positioning with bottom set to 0.
(Keep in mind that changing your li element to a block-level item could potentially impact other parts of your layout)
For centering, you can either make the span element fill the width and use text-align: center, use margin: 0 auto; (if the span element has been turned into a block-level element with a specific width), or utilize the left and right 0 trick