TextEdit User Guide
HTM or HTML is short for Hypertext Markup Language. It’s a file format used for containing HTML language which constructs a web page. This file format is based on markup codes intended to be used in Web Browsers. HTM contains formatted text, images, and other elements in a way that can be translated with web browsers. If you want to open a MHTML file on a Mac and have MS Office installed, just rename the file extension to eml. Microsoft Entourage can open the file with images and stuff. There is also an extension for Mozilla and Firefox to open web archives. Sadly it just works on Windows and Linux. On Mac it only works about 50%.
You can use TextEdit to edit or display HTML documents as you’d see them in a browser (images may not appear), or in code-editing mode.
Note: By default, curly quotes and em dashes are substituted for straight quotes and hyphens when editing HTML as formatted text. (Code-editing mode uses straight quotes and hyphens.) To learn how to change this preference, see New Document options.
Open Html File In Firefox
Create an HTML file
In the TextEdit app on your Mac, choose File > New, then choose Format > Make Plain Text.
Enter the HTML code.
Choose File > Save, type a name followed by the extension .html (for example, enter index.html), then click Save.
When prompted about the extension to use, click “Use .html.”
View an HTML document
In the TextEdit app on your Mac, choose File > Open, then select the document.
Click Options at the bottom of the TextEdit dialog, then select “Ignore rich text commands.”
Click Open.
Always open HTML files in code-editing mode
In the TextEdit app on your Mac, choose TextEdit > Preferences, then click Open and Save.
Select “Display HTML files as HTML code instead of formatted text.”
Change how HTML files are saved
Set preferences that affect how HTML files are saved in TextEdit.
In the TextEdit app on your Mac, choose TextEdit > Preferences, then click Open and Save.
Below HTML Saving Options, choose a document type, a style setting for CSS, and an encoding.
Select “Preserve white space” to include code that preserves blank areas in documents.
If you open an HTML file and don’t see the code, TextEdit is displaying the file the same way a browser would (as formatted text).