Exploring defer support in XMLParserScriptRunner for SVG Scripts

[Virali Purbey]

Hi,
I'm Virali from the SVG team at Microsoft.

I've been contributing to Chromium for the past year, and my recent work includes adding support for the async attribute in SVG <script> elements. I also initiated an Intent to Ship (I2S) on blink-dev regarding this, which drew significant attention from reviewers, as we currently support only the async attribute, and not defer.

We've received a lot of interest from the Chromium community in supporting both attributes together. Based on this, we plan to implement defer support in XMLParserScriptRunner this quarter, as the related bugs have received considerable upvotes.

In my initial investigation, I observed that supporting defer and type=module in XMLParserScriptRunner could lead to redundant code between the HTML and XML parser script runners.

I also connected with the feature owners, who mentioned ongoing discussions around whether to merge the script runners or extend the functionality within XMLParserScriptRunner.

I’d love to understand more about this and hear your thoughts. In the meantime, I’m deepening my understanding and working on a design document to explore potential approaches to supporting defer in XML scripts, including their trade-offs.

Looking forward to hearing from you.

Thank you,
Virali

[Virali Purbey]

Hey all,

Just bumping up this email as a gentle reminder in case it was missed. Looking forward to hearing your thoughts on defer support in XMLParserScriptRunner.

Thanks,

Virali

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From: Virali Purbey <virali...@microsoft.com>
Sent: Thursday, August 07, 2025 22:30
To: loadi...@chromium.org <loadi...@chromium.org>
Cc: Vinay Singh <vinay...@microsoft.com>; Ragvesh Sharma <rags...@microsoft.com>; Daniel Clark <dan...@microsoft.com>; Akshay Prabhu <apr...@microsoft.com>; Mike Jackson (EDGE) <mjac...@microsoft.com>
Subject: Exploring defer support in XMLParserScriptRunner for SVG Scripts

[Yoshisato Yanagisawa]

We have team members who would likely want to weigh in on this topic. They are currently out of office for the summer/Obon holidays. Let's revisit this discussion with them next week.

2025年8月12日(火) 11:39 'Virali Purbey' via loading-dev <loadi...@chromium.org>:

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[Virali Purbey]

Hey,

 

I hope everyone is back after spending a wonderful and refreshing vacation. Could we please revisit this topic and continue our discussion?

I’m eager to understand more about how we plan to enhance functionality in XMLParserScriptRunner, and whether any conversations are already underway in this regard. I’d really appreciate hearing more from the team.

Thank you
Virali

 

From: Yoshisato Yanagisawa <yyana...@chromium.org>
Sent: 12 August 2025 09:42
To: Virali Purbey <virali...@microsoft.com>
Cc: loadi...@chromium.org; Vinay Singh <vinay...@microsoft.com>; Ragvesh Sharma <rags...@microsoft.com>; Daniel Clark <dan...@microsoft.com>; Akshay Prabhu <apr...@microsoft.com>; Mike Jackson (EDGE) <mjac...@microsoft.com>
Subject: [EXTERNAL] Re: Exploring defer support in XMLParserScriptRunner for SVG Scripts

 

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[Virali Purbey]

Hi team,

We have drafted a proposal document for the problem statement. Here is the link: https://docs.google.com/document/d/15qFv33dwAxloXAeMa3ZEuw5iGZxAwXKcBry_gFCq1UA/edit?usp=sharing

Please have a look and let’s discuss the approaches for our next steps. I look forward to hear your feedback on the same.

Thanks
Virali

[Kouhei Ueno]

Hi Virali,

Thanks for reaching out with your detailed proposal regarding the defer attribute.

After discussing this with the owners of the script + parser/XML part of the codebase, we have decided that we cannot support the implementation of defer in XML documents at this time.

Our primary concern is that this would involve significant code rearchitecturing, as you outlined on the document. On the other hand, standard bodies' work on XHTML ceased back in 2009. Our general strategy for legacy technologies like this is to maintain the existing support for compatibility, especially for enterprise use cases, but to avoid adding new capabilities. For SVG, we are aware of  an open issue at SVGWG tracker, but we see the lack of activity on it for over a decade as a signal that this isn't a priority for the broader web community.

From a codebase perspective, the proposal touches a particularly complex part of Chromium that we are seeking to simplify over time, not expand. While we recognize there is some developer interest, we believe the significant maintenance cost and added complexity outweigh the benefits. Our engineering efforts are better focused on the modern HTML parser.

We appreciate you taking the time to write up the proposal and start a design document, and we would be happy to collaborate on features for the modern web platform in the future.

Best,

Kouhei

2025年8月21日(木) 18:50 'Virali Purbey' via loading-dev <loadi...@chromium.org>:

To view this discussion visit https://groups.google.com/a/chromium.org/d/msgid/loading-dev/SI2P153MB0507D5CDB20EC4AE728BC27BC132A%40SI2P153MB0507.APCP153.PROD.OUTLOOK.COM.