Only what you touch is rewritten in the file; everything else (styles, headers and footers, images) stays exactly as it was. Tables can be edited: touch one and it is rebuilt with the borders and widths of the original. A table inside a table is left alone — this version cannot write that back reliably.
Hyperlinks are preserved but not editable. Paragraphs containing fields, bookmarks or tracked changes are locked (grey edge) and go back into the file completely unchanged. Pasting inserts plain text only.
Open a Word file (.docx), edit the text with formatting (bold, italic, headings, find & replace) and save it again as .docx — no Office needed, 100% in your browser. Whatever you don't touch (styles, headers and footers, images) stays unchanged in the file.