Эксперименты с обводкой в CSS
Общие стили
.polygon,.trapezoid { 
 margin:0; padding:0; background:none; 
}

.polygon { 
 width:10em; height:10em; border:none; display:inline-block; text-align:center; 
}

.trapezoid {
 display:inline-block; width:1px; height:0; margin:0 auto;
 border-left:5em solid transparent; border-right:5em solid transparent;
}

.polygon .aa { 
 border-bottom:10em solid;border-top:none; 
}

.polygon .oo,.polygon .rr { 
 border-top:10em solid;border-bottom:none; 
}

.polygon .rr { 
 border-left:none; border-right:none; width:10em; 
}
Фигура HTML CSS
<a class="polygon r3">
 <b class="trapezoid aa"></b>
</a>
.polygon.r3 { 
 height:8.66em; 
}
.polygon.r3 .trapezoid { 
 border-width:8.66em 5em; 
}
<a class="polygon r4">
 <b class="trapezoid aa"></b>
</a>
.polygon.r4 .trapezoid { 
 border-left:none; 
 border-right:none; 
 width:10em; 
}
<a class="polygon r5">
 <b class="trapezoid aa"></b>
 <b class="trapezoid oo"></b>
</a>
.polygon.r5 { 
 height:9.51em; 
}

.polygon.r5 .aa { 
 border-bottom-width:3.63em; 
}

.polygon.r5 .oo { 
 border-width:5.88em 1.91em; 
 width:6.18em; 
}
<a class="polygon r6">
 <b class="trapezoid aa"></b>
 <b class="trapezoid oo"></b>
</a>
.polygon.r6 { 
 height:8.66em; 
}

.polygon.r6 .aa,
.polygon.r6 .oo { 
 border-width:4.33em 2.5em; 
 width:5em; 
}
<a class="polygon r8">
 <b class="trapezoid aa"></b>
 <b class="trapezoid rr"></b>
 <b class="trapezoid oo"></b>

</a>
.polygon.r8 { 
 height:10em; 
}

.polygon.r8 .rr { 
 border-top-width:4.14em; 
}

.polygon.r8 .aa,
.polygon.r8 .oo { 
 width:4.14em; 
 border-width:2.93em; 
}