What does metric-compatible mean?

It means a different typeface designed so that every character occupies exactly the same width as the one it replaces. Carlito stands in for Calibri, Caladea for Cambria, Liberation Serif for Times New Roman. The shapes differ slightly; the line breaks and page count do not, which is what people actually notice.

Does this editor ship the substitute fonts?

No. It names them in the fallback stack, so a machine that has Carlito — most Linux systems and anywhere LibreOffice is installed — uses it automatically for a Calibri document. On a machine without it the browser picks its own substitute, which keeps the shapes readable but not the metrics.

Why did my document gain a page?

Because the substitute is wider. Each line wraps a word earlier, each paragraph gains a line, and after twenty paragraphs the last one has been pushed onto a new page. Nothing in the document changed; the same text was measured with a different ruler.

Is embedding fonts in the .docx an option?

Word can embed fonts in a .docx and this editor does not write them, partly because most commercial fonts forbid it in their licence. The reliable cross-platform answer is a PDF with the fonts embedded, which is what PDF was invented for.

Why Your Fonts Change Between Word and Other Editors

Your document arrives looking different and a page longer. Here is why font substitution does that, which families are safe, and how to make a file that cannot shift.