What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

HTML tags written using some uppercase letters (i.e. <H1> or <Span>) will stop rendering their children. This happens because those tags are considered components by the maybeComponent() function, so after applying these changes their children will be considered the contents of its default slot. As native HTML tags doesn't have slots, they won't be rendered. This is explained in more detail in the attached video.

The recommendation in this case is to write the standard HTML tags always in lowercase (in fact, in Vue 3 a "failed to resolve component" warning is displayed when an uppercase tag is used, even when it's still rendered).

The PR fulfills these requirements:

• It's submitted to the dev branch for v2.x (or to a previous version branch), not the master branch
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:

P.S.: I just realized I didn't show the fixed version working as expected in the video (I forgot to modify the input during the demo). If you find it necessary, I can upload another video showing it 🙂 thank you very much!