First, I use a font called "Migu 2M" in Emacs.

When I run M-x htmlfontify-buffer in one buffer, the exported HTML contains
lines like:

```
body, pre { text-decoration: none;  font-family: Migu 2M;  font-stretch:
normal;  font-weight: 500;  font-style: normal;  color: #ffffff;
 background: #000000;  font-size: 15pt; }
```

Standards-compliant web browsers should ignore this font-family.

MDN Web Docs says:
https://developer.mozilla.org/en-US/docs/Web/CSS/font-family

> Valid family names
> Font family names must either be given quoted as strings, or unquoted as
a sequence of one or more identifiers. This means that punctuation
characters and digits at the start of each token must be escaped in
unquoted font family names.
>
> It is a good practice to quote font family names that contain white
space, digits, or punctuation characters other than hyphens.

An unquoted font-family is valid as long as it doesn't start with a digit,
but MDN Web Docs also says:

> The following example is technically valid but is not recommended:
> font-family: Gill Sans Extrabold, sans-serif;

So it makes sense to quote all font-family.

The attached patch file fixes that issue. Thank you for reading.