XSLT + BEM

<!-- XML -->
<?xml version="1.0" encoding="utf-8"?>
<?xml-stylesheet href="bem.xsl" type="text/xsl" ?>
<root xmlns:m="bem">
<b-gallery m:type="interiors" m:size="small">
    <thumb>
        <b-image src="" m:size="small"></b-image>
    </thumb>
    <thumb>
        <b-image src="" m:size="small"></b-image>
    </thumb>
    <thumb>
        <b-image src="" m:size="small"></b-image>
    </thumb>
</b-gallery>
</root>

<!-- XSL -->
<?xml version="1.0" encoding="utf-8"?>
<xsl:stylesheet version="1.0"
    xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
    xmlns:func="http://exslt.org/functions"
    xmlns:exsl="http://exslt.org/common"
    xmlns:bem="bem"
    xmlns:m="bem:m"
    extension-element-prefixes="func">

<xsl:output method="html" version="4.0" encoding="UTF-8"/>

    <xsl:template match="*" mode="bem">
        <div>
            <xsl:apply-templates select="." mode="m"/>
            <xsl:apply-templates select="*" mode="bem"/>
            <xsl:value-of select="text()"/>
        </div>
    </xsl:template>

    <xsl:template match="*" mode="m">
        <xsl:copy-of select="@*[namespace-uri() = '' and name() != 'class']"/>
        <xsl:choose>
            <xsl:when test="starts-with(local-name(), 'b-')">
                <xsl:apply-templates select="." mode="b"/>
            </xsl:when>
            <xsl:otherwise>
                <xsl:apply-templates select="." mode="e"/>
            </xsl:otherwise>
        </xsl:choose>
    </xsl:template>

    <xsl:template match="*" mode="b">
        <xsl:attribute name="class">
            <xsl:value-of select="local-name()"/>
            <xsl:apply-templates select="@m:*">
                <xsl:with-param name="name" select="local-name()"/>
            </xsl:apply-templates>
        </xsl:attribute>
    </xsl:template>

    <xsl:template match="*" mode="e">
        <xsl:param name="name">
            <xsl:variable name="ancestors" select="ancestor::*[starts-with(local-name(), 'b-')]"/>
            <xsl:value-of select="local-name($ancestors[position() = last()])"/>
            <xsl:text>__</xsl:text>
            <xsl:value-of select="local-name()"/>
        </xsl:param>

        <xsl:attribute name="class">
            <xsl:value-of select="$name"/>
            <xsl:apply-templates select="@m:*">
                <xsl:with-param name="name" select="$name"/>
            </xsl:apply-templates>
        </xsl:attribute>
    </xsl:template>

    <xsl:template match="@m:*">
        <xsl:param name="name"/>
        <xsl:text> </xsl:text>
        <xsl:value-of select="$name"/>
        <xsl:text>_</xsl:text>
        <xsl:value-of select="local-name()"/>_<xsl:value-of select="."/>
    </xsl:template>

    <xsl:template match="@*"/>

    <func:function name="bem:block">
        <xsl:param name="block"/>
        <func:result>
            <xsl:apply-templates select="exsl:node-set($block)" mode="bem"/>
        </func:result>
    </func:function>
</xsl:stylesheet>