It should be `{. xxxx=yyy }`, not `{_ xxxx=yyy }`.
Only test/test-element-attributes-markdown.html were using `{_`,
simply in order to test the ability, for the user, to define a _different_ pattern.
```html
<section data-markdown>
<script type="text/template">
- - Item 1 {_class="fragment" data-fragment-index="2"}
- - Item 2 {_class="fragment" data-fragment-index="1"}
+ - Item 1 {.class="fragment" data-fragment-index="2"}
+ - Item 2 {.class="fragment" data-fragment-index="1"}
</script>
</section>
```