So you want to embed an SVG on your website and add this to your HTML:

<object class="page" data="input_svg.svg" type="image/svg+xml"></object>

In my case this is an Affinity Designer SVG export.

But now we realize that the fonts don't work. That's because your pretty SVG doesn't tell the browser where to find these fonts. Usually you can fix this with CSS like this, making sure you actually host the .woff file at the specified location:

@font-face {
    font-family: 'Candara';
    src: url('/vendor/fonts/Candara-Regular.woff')format('woff');
    font-weight: normal;
    font-style: normal;
}

Unfortunately this CSS can't just be included in your HTML document, it has to be part of the SVG itself. This is how the original SVG looks like:

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
<svg [...]>
    [...]
</svg>

After the injection we need it to look like this:

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
<svg [...]>
    <defs>
        <style type="text/css"><![CDATA[
            @font-face {
                font-family: 'Candara';
                src: url('/vendor/fonts/Candara-Regular.woff')format('woff');
                font-weight: normal;
                font-style: normal;
            }
        ]]></style>
    </defs>

    [...]
</svg>

All Hail sed, a stream editor!

So, how do we do this?

We have the font-loading CSS in the file fonts.css. Our first challenge lies in surrounding this CSS with the required SVG stuff.

Let's do this with sed and start with creating the file fonts_css_template.txt:

<defs>
    <style type="text/css"><![CDATA[
OHtFGivqhAswi
    ]]></style>
</defs>

As you can see the (randomly chosen) string OHtFGivqhAswi needs to be replaced with the CSS.

sed "/OHtFGivqhAswi/e cat fonts.css" fonts_css_template.txt | \
    sed 's/OHtFGivqhAswi//' \
    > final_text_to_include_in_svg.txt

1. The first sed command reads fonts.css and puts it right before OHtFGivqhAswi;
2. the second deletes OHtFGivqhAswi. This only works because OHtFGivqhAswi is on its own line.
3. We pipe the output into final_text_to_include_in_svg.txt. This is what needs to be injected in the SVG.

Commence the Injection

sed -E 's/(<svg [^>]+>)/\1\ncI0WWZKD2UKEj\n/' input_svg.svg | \
    sed "/cI0WWZKD2UKEj/e cat final_text_to_include_in_svg.txt" | \
    sed 's/cI0WWZKD2UKEj//' \
    > output_svg.svg

1. Our first command finds the opening svg tag and appends the random string cI0WWZKD2UKEj, making sure it is on its own line.
2. This is followed by exactly what we've done before: prepend what needs to be injected,
3. replace cI0WWZKD2UKEj and
4. pipe the output where it is needed.

You don't need that ginormous web framework for simple things—some CSS and sed does the trick just fine. Perhaps, first take a look at what the GNU toolbox offers when solving the next challenge you come across.

The Final Script

#!/bin/bash
set -euo pipefail
DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" >/dev/null 2>&1 && pwd)"

FONTS_CSS_TEMPLATE="$DIR/fonts_css_template.txt"
FINAL_TEXT_TO_INCLUDE_IN_SVG="$DIR/final_text_to_include_in_svg.txt"
FONTS_CSS_FILE="$DIR/fonts.css"

echo "converting $1 to $2"

sed "/OHtFGivqhAswi/e cat $FONTS_CSS_FILE" "$FONTS_CSS_TEMPLATE" | \
    sed 's/OHtFGivqhAswi//' \
    > "$FINAL_TEXT_TO_INCLUDE_IN_SVG"

sed -E 's/(<svg [^>]+>)/\1\ncI0WWZKD2UKEj\n/' "$1" | \
    sed "/cI0WWZKD2UKEj/e cat $FINAL_TEXT_TO_INCLUDE_IN_SVG" | \
    sed 's/cI0WWZKD2UKEj//' \
    > "$2"

Save this as load_fonts_in_css.sh, run chmod +x ./load_fonts_in_css.sh and ./load_fonts_in_css.sh input_svg.svg output_svg.svg.