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$ appstreamcli --version
AppStream version: 0.15.5

$ cat lorem.metainfo.xml 
<?xml version="1.0" encoding="UTF-8"?>
<component>
    <id>io.sourceforge.lorem</id>
    <name>Lorem</name>
    <summary>Lorem Ipsum</summary>
    <project_license>GPL-3.0-or-later</project_license>
    <metadata_license>FSFAP</metadata_license>
    <description>
        <p>
            Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor
            incididunt ut labore et dolore magna aliqua.
        </p>
    </description>
    <releases>
        <release date="2022-10-01" version="0.1.0">
            <description>
                <p>
                    <code>--syslog</code> command line option is introduced.
                </p>
            </description>
        </release>
    </releases>
</component>

$ appstreamcli validate lorem.metainfo.xml 
I: io.sourceforge.lorem:16: description-first-word-not-capitalized

✔ Validation was successful: infos: 1

appstreamcli complains description-first-word-not-capitalized, while the first word of the description is --syslog, it can't be capitalized. appstreamcli should not complain if the first character of the description is not a letter.