I've build an example app in svelte and sapper with same ui and figure out, the content appears different:
I've checked the css, but it should not be any differences.
The code from https://gitlab.com/oceanstack/ui5-sapper and https://gitlab.com/oceanstack/ui5-svelte
The problem is your HTML is not the same.
Svelte:
<ui5-label id="myLabel" for="myInput" required show-colon="">First name</ui5-label>
Sapper:
<ui5-label id="myLabel" for="myInput" show-colon="">First name</ui5-label>
Notice that Svelte has the required
attribute and Sapper does not.
If you go into the browser development tools on the Sapper page and add the required
attribute to the label, you'll notice that all of a sudden, the ellipsis is removed and that your labels will match.