`sanitizeHtml` in src/sanitizer.js is causing visual diff JS errors (started with prod push)

See discussion at https://github.com/ampproject/amphtml/pull/13333#issuecomment-363918699

This started failing on master with this error:

/home/travis/.rvm/gems/ruby-2.4.1/gems/poltergeist-1.17.0/lib/capybara/poltergeist/browser.rb:384:in `command': One or more errors were raised in the Javascript code on the page. If you don't care about these errors, you can ignore them by setting js_errors: false in your Poltergeist configuration (see documentation for details). (Capybara::Poltergeist::JavascriptError)
Removing "class" attribute with invalid value in <amp-img class="i-amphtml-element i-amphtml-notbuilt amp-notbuilt">.
Removing "class" attribute with invalid value in <amp-img class="i-amphtml-element i-amphtml-notbuilt amp-notbuilt">.
    at https://cdn.ampproject.org/v0.js:2 in pa
Removing "class" attribute with invalid value in <amp-img class="i-amphtml-element i-amphtml-notbuilt amp-notbuilt">.
Removing "class" attribute with invalid value in <amp-img class="i-amphtml-element i-amphtml-notbuilt amp-notbuilt">.
    at https://cdn.ampproject.org/v0.js:2 in pa
Removing "class" attribute with invalid value in <amp-img class="i-amphtml-element i-amphtml-notbuilt amp-notbuilt">.
Removing "class" attribute with invalid value in <amp-img class="i-amphtml-element i-amphtml-notbuilt amp-notbuilt">.
    at https://cdn.ampproject.org/v0.js:2 in pa
Removing "class" attribute with invalid value in <amp-img class="i-amphtml-element i-amphtml-notbuilt amp-notbuilt">.
Removing "class" attribute with invalid value in <amp-img class="i-amphtml-element i-amphtml-notbuilt amp-notbuilt">.
    at https://cdn.ampproject.org/v0.js:2 in pa
	from /home/travis/.rvm/gems/ruby-2.4.1/gems/poltergeist-1.17.0/lib/capybara/poltergeist/browser.rb:39:in `visit'
	from /home/travis/.rvm/gems/ruby-2.4.1/gems/poltergeist-1.17.0/lib/capybara/poltergeist/driver.rb:99:in `visit'
	from /home/travis/.rvm/gems/ruby-2.4.1/gems/capybara-2.17.0/lib/capybara/session.rb:274:in `visit'
	from build-system/tasks/visual-diff.rb:288:in `block (2 levels) in generate_snapshots'
	from build-system/tasks/visual-diff.rb:281:in `each'
	from build-system/tasks/visual-diff.rb:281:in `block in generate_snapshots'
	from build-system/tasks/visual-diff.rb:275:in `each'
	from build-system/tasks/visual-diff.rb:275:in `generate_snapshots'
	from build-system/tasks/visual-diff.rb:250:in `run_visual_tests'
	from build-system/tasks/visual-diff.rb:433:in `main'
	from build-system/tasks/visual-diff.rb:441:in `<main>'

[rsimha]

@choumx I’ve just switched the visual tests from phantomJS to headless chrome.

[rsimha]

Yeah, we use PhantomJS. See the link under https://github.com/ampproject/amphtml/blob/master/contributing/DEVELOPING.md#visual-diff-tests.

I guess there are two questions to answer:

1. Why is this failing on PhantomJS?
2. Why did a prod push affect the behavior of a locally built runtime?