The Unicode specification states that a precomposed character like LATIN SMALL LETTER A WITH DIAERESIS (U+00E4) should have the same appearance and meaning when displayed, as the decomposed character sequence LATIN SMALL LETTER A (U+0061) and COMBINING DIAERESIS (U+0308).
On web pages that use web fonts, characters with diacritic (like: ä, ñ, é, å, ç, î, …) are not rendered properly, if they are encoded as decomposed character sequence.
You can find a test case under the following url:
Safari 12 on Mac and iOS do not render these characters properly, as well as Safari Technology Preview Release 85. (Chrome 75 renders decomposed character sequences properly.)