“I wonder if any these tests could be made headless. I think there's a potential to do so. Yet it's out of scope of migration, I guess.” [1]

It refers to tests open-sourced by JDK-8315876.

The list of tests:
test/jdk/javax/swing/text/html/CSS/bug4174871.java
test/jdk/javax/swing/text/html/CSS/bug4174874.java
test/jdk/javax/swing/text/html/CSS/bug4284162.java
test/jdk/javax/swing/text/html/CSS/bug4764897.java
test/jdk/javax/swing/text/html/HTMLDocument/bug4209280.java

[1] https://github.com/openjdk/jdk/pull/15769#pullrequestreview-1630692732