Rich HTML (slot) for sl-input placeholder

[brettwillis]

Currently, SlInput.placeholder only supports a string value due to it being assigned directly to the HTMLInputElement.placeholder attribute.

However consider a scenario when you want rich HTML, for example a search field with a styled <span><kbd>/</kbd>Search&hellip;</span> placeholder.

This would require a "placeholder" slot in the sl-input, and ofcourse some non-native way of rendering a slotted HTML element as a placeholder.

[claviska]

This could be useful, but we'd need to add it to all form controls. We'd also need to clearly call out that only presentation content should be slotted here, as interactive content will cause accessibility issues.

Prior art can be found right here on GitHub.

[claviska]

This won't be as simple as I was hoping, because we're passing the placeholder through to the <input>.

I'd classify this as a "nice to have" but unlikely to be something we'll get to anytime soon due to the cost/benefit of making this work for all affected form controls. We'd probably have to add slot detection and a faux placeholder and juggle between using native text placeholder and the custom HTML/faux placeholder.

I'd be open to a PR if someone wants to take a stab at a PR, but I suspect the amount of code and complexity to achieve this will be more than we want to maintain.