Update Fx148 relnotes to include initial about:blank

[vinhill]

Description

Quoting from the change:

The initial about:blank document now loads synchronously. A browsing context's first navigation may resolve to about:blank (for example, when the initial URL is empty or explicitly set to about:blank). In these cases, Firefox no longer replaces the initial empty document with a second, asynchronously loaded one, and instead fires the load event synchronously on the initial document.

Motivation

Changing the timing of about:blank loads solves a long-standing web-compat issue. But we've seen web pages or addons break, so developers might want to know about this change.

To name some examples of what changed:

• window.open("about:blank").onload = handler the handler won't run anymore because load already fired
• The same with ifr = document.createElement("iframe"); document.body.append(ifr); ifr.onload = handler;
• If an empty iframe is appended and synchronously removed, declarative extension content scripts could execute in it anyway.
• ifr = document.createElement("iframe"); ifr.onload = handler; document.body.append(ifr); ifr.src = other will observe two load events now because src was set too late.

Related issues and pull requests

• https://bugzilla.mozilla.org/show_bug.cgi?id=543435
• https://www.firefox.com/en-US/firefox/148.0beta/releasenotes/

[github-actions]

Preview URLs (1 page)

• /en-US/docs/Mozilla/Firefox/Releases/148

External URLs (1)

URL: /en-US/docs/Mozilla/Firefox/Releases/148
Title: Firefox 148 release notes for developers (Beta)

• https://bugzil.la/543435 (1 time) (Note! This may be a new URL 👀)

[pepelsbey]

Hey! Thank you, it looks very good. The only question I have is about the section. Looking back to how we used the HTML section, it usually has something to do with markup, elements, attributes, etc.

This change looks a bit different. More like API or DOM. Would you consider other sections for this change?

[vinhill]

Hey!
I think HTML is the better fit as this is about the document/navigation lifecycle. I only see DOM as subsection of API and this is not an API change.