Our 2019 Contributions to Web Platform Interoperability


Editor’s Note: the following post was written by Frédéric Wang, Partner at Igalia

For the third year in a row, the AMP Project has worked with Igalia to address bug reports and enhancement requests from the Web community. Typical tasks include bug triaging, debugging and analysis, writing tests, landing patches, and discussions with the different actors of the Web platform. This has not only been instrumental to significantly speed up the goals of the AMP project, but has also been generally beneficial to all the developers and users of the Web platform.

This blog post describes our activities for this year, focusing on WebKit/iOS bugs and features. However, our Web platform interoperability effort occasionally extends to other browser communities and standardization groups as well.

Resize Observer

Intersection Observer shipped in iOS 12.2 thanks to Google Engineer Ali Juma. This is a new JavaScript API to asynchronously watch changes in the intersection of a target element with an ancestor element or with a top-level document’s viewport.

Resize Observer is a similar API  which we implemented earlier this year under a preference flag. This JavaScript API allows Web developers to asynchronously observe when an element is resized, regardless of why. It is available in iOS 13 beta and the corresponding preference flag has been turned on by default in trunk, so we expect this feature to be available in the iOS 13 release. (Update 01/28/2020: ResizeObserver is now part of Safari Technology Preview)


Apple took over the task we started last year to make <iframe> elements scrollable and this behavior is now enabled by default in the latest iOS 13 version. Other bug fixes involving scrollable elements landed for scroll-snap-align, scrollability after resize find-in-page indicator after scroll as well as for various iOS 13 regressions related to scroll flickering and jittering.

Enhancements have been implemented for the ScrollIntoViewOptions parameter of scroll APIs. Support for logical scroll alignment is shipped in iOS 13. We also continued our efforts to support the scroll-behavior IDL parameter and CSS property and we expect to complete this in the next semester. While working on this, we also detected and fixed Chrome bugs related to the scrollIntoView() method, including cases when a scrollbar is present or when the scroller uses a non-default writing mode.

An old browser interoperability issue for users relates to inconsistent values of scrollLeft, scrollTop and similar APIs and one of our important achievements has been to ensure more reliable and standard behavior happens when setting or getting scroll coordinates. We introduced an option to make Chrome use standard values in non-default writing modes and plan to ship it, after ensuring that it won’t cause serious breakage. Similarly, Apple decided to enable our 2018 changes for the viewport scroller on all WebKit ports.

Besides usual bug fixes, we began implementing other interesting scrolling features, including overscroll customization and overscroll-behavior which are powerful APIs for web developers to control what happens when a scroller reaches its boundaries. We expect more progress on this next year.

Resource Loading

Another exciting goal is to give more power to Web authors to control loading behavior. In particular, this allows the ability to control privacy and optimize page layout.

The referrerpolicy attribute has been implemented to specify how much referrer information should be included within the requests associated to an HTML element loading resources. This is only implemented for the <iframe> and <script> elements and is available in iOS 13 under an experimental feature flag. We will continue to talk to Apple to see when this can be enabled by default or implemented for other elements.

The imagesrcset and imagesizes attributes on <link rel=preload> have also been implemented and are available in iOS 13 under an experimental feature flag. These attributes give the possibility to preload a responsive image represented by an <img> element with relevant sizes and srcset attributes and optimize the selection of the appropriate size for the user device.

We also started to submit patches to support the lazyload attribute on the <img> and <iframe> elements. These attributes enable Web authors to indicate whether it is a good idea to lazily load the element content (e.g. if they are not visible in the viewport until the user scrolls to them) or if their content should instead be loaded right away. These hints are very helpful for browsers to optimize loading of resources.

Finally, we made an experimental support for the intrinsic size attribute in WebKit. This proposal is intended to help browsers to determine aspect ratio or size of an image before its content is actually loaded, in order to avoid extra post-load reflow. This proposal has been superseded by a pure CSS-based approach addressing the same use case. Our experiment was useful for discussions among browser vendors and within the CSS WG and we plan to rewrite our patch to instead implement the CSS-based approach in WebKit.


Collaboration between the AMP project and Igalia to advance the state of the Web Platform has been very successful. There are several pending tasks and new ideas to work on so we look forward to continuing with this effort next year!