We have a custom attribute that we've made for local css modules (using webpack's css-loader's `modules` option) which maps the class in the css definition to the local identifier that's generated. The custom attribute is used like so:

``` html

My Heading

```

This works great; it takes the values in css-class and gets the mapped identifier, and then we apply the class to it accordingly.

The problem is, we also want to be able to apply the attribute to the `` tag for the component's root level element. This works, but if we also apply a css-class attribute when the component is used, it is overridden instead of combined. For classes, the values are combined, and we'd like the same for our custom attribute. Is there a setting that I'm missing? I can't seem to find it when going through the source.

Thanks!

No files, tests, or entry points are named. Start by locating the templating implementation for custom attributes on template and component elements, then compare it with the existing class-value merging behavior; done means the requested custom attribute values combine consistently or the supported setting is identified and covered by a regression test.

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