ADDITIONAL SYSTEM INFORMATION :
Windows 10 (1809)
java version "11.0.5" 2019-10-15 LTS
Java(TM) SE Runtime Environment 18.9 (build 11.0.5+10-LTS)
Java HotSpot(TM) 64-Bit Server VM 18.9 (build 11.0.5+10-LTS, mixed mode)

A DESCRIPTION OF THE PROBLEM :
NumberFormatException is thrown for CSS font-size:
font-size: 100%;

The class CSS.FontSize (package javax.swing.text.html) supports %-values, however the method parse:2875, CSS$LengthUnit (javax.swing.text.html) has the "if" statement:

2875: if (length > 0 && value.charAt(length - 1) == '%') {
                try {
                    this.value = Float.valueOf(value.substring(0, length - 1)). floatValue() / 100.0f;
                    ...
                }
                catch (NumberFormatException nfe) { }
            }
            if (length >= 2) {
            ...
so, if we are able to parse "100%", the parsing is not interrupted, it goes further and fails with the exception

REGRESSION : Last worked in version 8

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
parse CSS font-size value. E.g.
   font-size: 100%;

if possible, please look at the implementation of the method parse in line 2875.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Parse the value without any exception
ACTUAL -
readJavaFormatString:2054, FloatingDecimal (jdk.internal.math)
parseFloat:122, FloatingDecimal (jdk.internal.math)
parseFloat:455, Float (java.lang)
valueOf:419, Float (java.lang)
parse:2914, CSS$LengthUnit (javax.swing.text.html)
<init>:2867, CSS$LengthUnit (javax.swing.text.html)
parseCssValue:2081, CSS$FontSize (javax.swing.text.html)
getInternalCSSValue:839, CSS (javax.swing.text.html)
addInternalCSSValue:824, CSS (javax.swing.text.html)
addCSSAttribute:527, StyleSheet (javax.swing.text.html)
handleValue:3342, StyleSheet$CssParser (javax.swing.text.html)
parseDeclaration:358, CSSParser (javax.swing.text.html)
parseDeclarationBlock:325, CSSParser (javax.swing.text.html)
parseRuleSet:272, CSSParser (javax.swing.text.html)
getNextStatement:178, CSSParser (javax.swing.text.html)
parse:153, CSSParser (javax.swing.text.html)
parse:3261, StyleSheet$CssParser (javax.swing.text.html)
addRule:314, StyleSheet (javax.swing.text.html)
createStyleSheet:89, JBHtmlEditorKit (com.intellij.util.ui)
configureHtmlKitStylesheet:93, StartupUiUtil (com.intellij.util.ui)
lambda$prepareAndStart$1:164, StartupUtil (com.intellij.idea)
run:-1, 1760817729 (com.intellij.idea.StartupUtil$$Lambda$63)
run:1736, CompletableFuture$AsyncRun (java.util.concurrent)
dispatch:313, InvocationEvent (java.awt.event)
dispatchEventImpl:776, EventQueue (java.awt)
run:727, EventQueue$4 (java.awt)
run:721, EventQueue$4 (java.awt)
doPrivileged:-1, AccessController (java.security)
doIntersectionPrivilege:85, ProtectionDomain$JavaSecurityAccessImpl (java.security)
dispatchEvent:746, EventQueue (java.awt)
pumpOneEventForFilters:203, EventDispatchThread (java.awt)
pumpEventsForFilter:124, EventDispatchThread (java.awt)
pumpEventsForHierarchy:113, EventDispatchThread (java.awt)
pumpEvents:109, EventDispatchThread (java.awt)
pumpEvents:101, EventDispatchThread (java.awt)
run:90, EventDispatchThread (java.awt)

CUSTOMER SUBMITTED WORKAROUND :
don't use %

FREQUENCY : always