cc @empijei @kele

Working as intended, from the documentation of the error:

// ErrBranchEnd: "{{if}} branches end in different contexts"
// Example:
// {{if .C}}<a href="{{end}}{{.X}}
// Discussion:
// Package html/template statically examines each path through an
// {{if}}, {{range}}, or {{with}} to escape any following pipelines.
// The example is ambiguous since {{.X}} might be an HTML text node,
// or a URL prefix in an HTML attribute. The context of {{.X}} is
// used to figure out how to escape it, but that context depends on
// the run-time value of {{.C}} which is not statically known.

{{.type}} may contain a " which ends the context early.

The example in the documentation has an {{if}} outside the tag and its {{end}} inside the tag (as far as we can tell). Hence the "different context". In my case, both {{if}} and {{end}} are inside the tag. {{if} branches are allowed and are useful around attributes. How else would you place attributes conditionally in a tag? The problem is with one specific attribute ("type") in the <script> element. I do not get the error for other <script> attributes or for a "type" attribute in other elements.

I am looking at the code (escape.go) now and I see the special treatment of the "type" attribute inside a <script> tag:
// If this is a non-JS "type" attribute inside "script" tag, do not treat the contents as JS.
but <script type="module"> is a valid javascript tag. See here: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Modules

This no longer affects me. If I want to add type="module" conditionally, I can do it differently. This works, for example, and it is better because the type attribute cannot have arbitrary values:

<script {{if .module}}type="module"{{end}}></script>

It turns out I don't need this conditional at all. JavaScript modules are imported differently in HTML than other JavaScript.