Try removing the space before the ‘X’ in tex4ht.env and using the following NSGMLS make file adjusted to your system.
all:    ${ext} 
 
html: 
        rm -f temp.html 
        rm -f fooo 
        ###################### ${name}.${ext} 
        sed ${sedDTD}  < ${name}.${ext}   > temp.html 
        ${nsgmls} -e 's/${dtdx}/nsgmls  ${dclx} temp.html/' \ 
                  -e 's/${dtd4}/nsgmls  ${dcl}  temp.html/' \ 
                  -e 's/${dtd32}/nsgmls ${dcl}  temp.html/'     > fooo 
        chmod 700 fooo 
        fooo  name=${name}  ext=${ext}           ########  ${name}.${ext} 
        #====================== ${name}.${ext} 
 
tmp: 
 
css: 
 
##################################################################### 
 
nsgmls = grep '<!DOCTYPE' ${name}.${ext} | sed 
 
dcl =  -s ${path}\/html.dcl 
dclx =  -wxml -wall -c ${path}\/catalog -s ${path}\/xml.dcl 
 
dtdx = <\!DOCTYPE .* "-\/\/W3C\/\/DTD XHTML 1.0 Transitional\/\/EN" 
dtd4 = <\!DOCTYPE .* "-\/\/W3C\/\/DTD HTML 4.0 Transitional\/\/EN" 
dtd32 = <\!DOCTYPE html PUBLIC "-\/\/W3C\/\/DTD HTML 3.2\/\/EN" 
 
sedDTD= -e 's/PUBLIC .*DTD XHTML.*"/SYSTEM    "${path}\/xhtml.dtd">/g'  \ 
        -e 's/PUBLIC .*DTD MATHML.*"/SYSTEM   "${path}\/mathml.dtd">/g' \ 
        -e 's/PUBLIC .*DTD HTML 3.2.*"/SYSTEM "${path}\/html32.dtd">/g' \ 
        -e 's/PUBLIC .*DTD HTML 4.0.*"/SYSTEM "${path}\/html4.dtd">/g'  \ 
        -e 's/"http:\/\/www.w3.org\/TR\/.*\/.*dtd">//'                  \ 
        -e 's/"http:..www.w3.org.pub.WWW.MarkUp.Wilbur.HTML32.dtd">//' 
 
path =  ....... 
 
#####################################################################