HTML: add html.build-preview stringparam

Author: ascholerChemeketa

xsl/pretext-html.xsl

@@ -104,13 +104,20 @@ along with MathBook XML.  If not, see <http://www.gnu.org/licenses/>.
 <!-- Or make a thin customization layer and use 'select' to provide overrides -->
 <!-- See more generally applicable parameters in pretext-common.xsl file      -->
 
-<!-- Fast HTML build that skips steps not needed for quickly rebuilding       -->
+<!-- Fast HTML builds that skip steps not needed for quickly rebuilding       -->
 <!-- individual pages in a document.                                          -->
-<!-- Skips rebuilding search index, theme, Runestone manifest,                -->
-<!-- moving external/generated files, ...                                     -->
-<!-- Should produce clean diffs against pages from non-incremental build      -->
+<!-- build-incremental: skips rebuilding search index, theme, Runestone       -->
+<!--   manifest, moving external/generated files,...                          -->
+<!--   Should produce clean diffs against pages from non-incremental build    -->
+<!-- build-preview, implies build-incremental and, when combined with         -->
+<!--   subtree, prunes the document to avoid processing nodes not part of     -->
+<!--   the output subtree.                                                    -->
+<!--   Will produce pages that are visually the same but have diffs when      -->
+<!--   compared to non-preview builds                                         -->
+<xsl:param name="html.build-preview" select="''"/>
+<xsl:variable name="b-build-preview" select="$html.build-preview = 'yes'"/>
 <xsl:param name="html.build-incremental" select="''"/>
-<xsl:variable name="b-build-incremental" select="$html.build-incremental = 'yes'"/>
+<xsl:variable name="b-build-incremental" select="$b-build-preview or $html.build-incremental = 'yes'"/>
 
 <!-- CSS and Javascript Directories -->
 <!-- These are convenience variables to specify file prefixes  -->