this helped me quite a bit already. I made this template:

    <xsl:template match="myhtml">
      <xsl:copy>
        <xsl:apply-templates select="child::node()"/>
      </xsl:copy>
    </xsl:template>

    <xsl:template match="*">
        <xsl:copy>
            <xsl:apply-templates select="@*|node()"/>
        </xsl:copy>
    </xsl:template>

    <xsl:template match="//mytag">
        blabla
    </xsl:template>

This does exactly what I want except for one thing: it also copies the myhtml tag. What am I doing wrong?

    <xsl:template match="myhtml">
        <xsl:apply-templates select="child::node()"/>
    </xsl:template>

    <xsl:template match="*">
        <xsl:copy>
            <xsl:apply-templates select="@*|node()"/>
        </xsl:copy>
    </xsl:template>

    <xsl:template match="//mytag">
        blabla
    </xsl:template>