Sebastian blogged about HTML5 WAC application support in Plasma, complete with a screencast, and it is looking really promising. Tizen, the combined effort of Intel and Samsung, is focusing on WAC which itself is nicely documented here. Plasma's flexibility in this area makes this a nice way to add to the number and variety of add-ons available for Plasma enabled devices.
Does this mean will Vivaldi have WAC support? the answer is "Yes, when the WAC script engine is ready". As it is self-contained in its own git repository and being added to the growing list of OBS packages available to those making Plasma Active images this will be trivial to ship as a post-launch update when it is ready.
In addition to getting WAC apps loading (something Marco did initial work on), Sebastian has also been implementing API access permissions and the Device Status API. The API permissions UI is important as we will almost certainly end up using this for native Plasma add-ons written in QML. There is a lot of other WAC API that needs implementing. If you're interested in hacking on this, check out the git repo with `git clone git://anongit.kde.org/scratch/sebas/plasma-scriptengine-wac`, or if you have set up git URL rewriting (which you should if you use KDE git repos as it saves on hassle) `git clone kde:scratch/sebas/plasma-scriptengine-wac`.
On a sidenote, I also fixed a bug that is visible in Sebastian's screencast where tooltips do not hide as they should when dragging a widget from the new QML based Add Widgets UI. Thinking of Sebastian's screencast, I wondered if anyone would have found it helpful had I recorded the process of fixing such a bug. It's an every day sort of thing to me, but would aspiring KDE hackers find it useful to see how those of us working on Plasma today accomplish such tasks? Let me know your thoughts in the comments.