What happens to accented characters?

They are folded to the closest plain letter, so Cafe Decor keeps its letters instead of becoming caf-cor. Stripping them is the commonest bug in slug generators.

Should I use hyphens or underscores?

Hyphens. Google treats a hyphen as a word separator and an underscore as a joiner, so top_10_tips reads as one word to a crawler and top-10-tips reads as three.

Why did my title produce nothing?

Because it had no characters a URL can use. A title written entirely in Japanese or emoji has no Latin equivalent to fold to, and the tool says so rather than handing back an empty string.

Slug Generator

Clean URL slugs, with accents folded properly.

Title

Separator

Clean URL slugs, with accents folded properly.

How to use it

Paste the title. Anything goes. Punctuation, accents and mixed case are all handled.
Pick a separator. Hyphens for anything public. Underscores only where a system requires them.
Copy the slug. Check it before you publish. A slug is permanent once it has been linked to.

When you would use this

A slug is the readable part of a URL, and it is worth getting right once because it is permanent: change it after anything links to the page and you have either a broken link or a redirect to maintain forever. The part that separates a working slug generator from a broken one is accents. Simply removing anything outside the ASCII range turns Cafe Decor into caf-cor, which is unreadable and unsearchable. Folding decomposes each letter into its base plus its accent and drops only the accent, so the letter survives. The same treatment handles the letters that have no accented form to decompose, such as the Polish crossed L and the German sharp S, which need their own mappings. Hyphens are the right separator for anything public.

Questions

What happens to accented characters?: They are folded to the closest plain letter, so Cafe Decor keeps its letters instead of becoming caf-cor. Stripping them is the commonest bug in slug generators.
Should I use hyphens or underscores?: Hyphens. Google treats a hyphen as a word separator and an underscore as a joiner, so top_10_tips reads as one word to a crawler and top-10-tips reads as three.
Why did my title produce nothing?: Because it had no characters a URL can use. A title written entirely in Japanese or emoji has no Latin equivalent to fold to, and the tool says so rather than handing back an empty string.