Commas in generated HTML

[Rich-Harris]

Describe the bug

When portions of markup are turned into HTML, attributes with interpolated literals (insofar as that's not an oxymoron) contain unwanted commas

To Reproduce

<div>
  <span class="a/{42}"/>
</div>

This needs to be compiled in production mode, since elements are never converted to static HTML in dev mode.

https://svelte.dev/repl/bf37bbf244594248b444a50ad3b95c73?version=3.6.10

Expected behavior

The chunks of the attribute should be concatenated without commas

Severity

Low, since attributes with interpolated literals are somewhat rare.

Additional context

In a case like this...

<script>
  const answer = 42;
</script>

<div>
  <span class="a/{answer}"/>
</div>

...Svelte should recognise that answer cannot change, and so it's ok to use innerHTML:

-span = element("span");
-attr(span, "class", span_class_value = "a/" + answer);
+div.innerHTML = `<span class="a/${answer}"></span>`;
// ...
-append(div0, span);