Make annotation of iframes opt-in

[robertknight]

It turns out that the client's iframe support is not yet robust enough (see #530) to enable it automatically for all same-origin iframes on arbitrary web pages.

To support the needs of EPUB viewers and others in the meantime while preventing problems on eg. pages with larger numbers of iframed ads, require the publisher to opt iframes into annotation by
adding the enable-annotation attribute to them.

Note that this doesn't fix the root issues described in #530 (comment) , but it does alleviate the symptoms on real-world websites until those issues are all addressed. Even when they are resolved, we will probably still want to use tougher heuristics before automatically enabling annotation for iframes because of the performance cost.

CC @jccr

[segdeha]

Rather than just requiring publishers to tag their iframes, could we not detect common ad sizes and ignore those frames? Most adverts are one of several sizes, most of which would be super unlikely to be a frame with content anyone wants to annotate.

[robertknight]

Rather than just requiring publishers to tag their iframes, could we not detect common ad sizes and ignore those frames?

Fair question. We already apply some heuristics like that, but they weren't good enough and we don't have a lot of bandwidth to validate anything we do come up with thoroughly at present for compatibility, performance etc. This approach is dumb & conservative but predictable.

[codecov]

Codecov Report

Merging #533 into master will increase coverage by <.01%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master     #533      +/-   ##
==========================================
+ Coverage   90.93%   90.93%   +<.01%     
==========================================
  Files         136      136              
  Lines        5426     5428       +2     
  Branches      945      945              
==========================================
+ Hits         4934     4936       +2     
  Misses        492      492

Impacted Files	Coverage Δ
src/annotator/util/frame-util.js	87.8% <85.71%> (+0.62%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4192f10...30475c0. Read the comment docs.

[robertknight]

Thanks for testing this out @sheetaluk 🙂