There is a new XHTML-compatible amp-bind attribute syntax: ampproject/amphtml#11115 via ampproject/amphtml#15408

Instead of:

<span class="foo" [class]="foo ? 'foo' : 'bar'">

We can now do:

<span class="foo" data-amp-bind-class="foo ? 'foo' : 'bar'">

What this means is that we now have an official way of representing amp-bind attributes in PHP's DOM internals. We can now eliminate AMP_DOM_Utils::get_amp_bind_placeholder_prefix() and AMP_DOM_Utils::restore_amp_bind_attributes() entirely, and AMP_DOM_Utils::convert_amp_bind_attributes() can be modified to just convert the bracketed syntax into the data-amp-bind-* syntax. There would be no need to restore the bracketed syntax when re-serializing. The result will be improved performance.